Glassing with water-based products

[Herb]

I am getting to a stage where - after some more weeks of sanding - I am ready to start glassing part of my BC <A href=http://aeneas.ps.uci.edu/edf/spit/pa230025.jpg> Spitfire </A>.

I have read through Jim Ryan's wonderful writeups on glassing & silkspan covering,

http://home.fuse.net/ryan/

The glassing pages mostly address using epoxy resins which I have used in the past with good results. Jim goes through the details of how to get a strong yet very lightweight product.

I have been considering trying again the water-based glassing method I have recently used in my <A href=http://aeneas.ps.uci.edu/edf/fa18/pa120001.jpg> F/A-18 </A> which seemed to work ok. I used satin Minwax <i>Polycrylic</i> from HomeDepot,

http://www.minwax.com/produse/clear/pucfpc1.htm

but I have tried <i>Varathane</i> water-based floor gloss polyurethane and seemed to like it less, heavier & not as nice & smooth a finish,

http://www.flecto.com/products/index.html



<i>Advantages of water-based products</i>:

Easy to apply & cleanup, no dilution necessary, can use & reuse applicators, multipler layers easy to apply, no epoxy mixing & associated mess, less toxic

<i>Disadvantages of water-based products</i>:

Polycrylic seems to take forever to cure to a hard shell consistency (2 weeks), not as hard as (laminating) epoxy perhaps, underlying wood can warp & deform due to the humidity.


Any suggestions? Other water-based harder/faster drying products I could try? Combining with primers to avoid possible warping?

[Jim Ryan]

I think you did a good job of weighing the relative merits of finishing epoxy versus water-based polyurethane. The giant scale crowd especially likes WBPU, because you can work so fast with it. Because of the types of structures they're covering, the potential for warping (due to the water content) is reduced, and they can paint on 2 or 3 coats in a day.

I personally prefer traditional glass and epoxy for the very reasons you sited. It's a known material that gives me absolutely consistent and predictable results. It seems to provide the best strength to weight ratio. It cures quickly so that I can proceed to priming and wet sanding. And I don't have to worry about it warping a lightweight sheet balsa stabilizer or vertical fin.

It seems to me that one big advantage of WBPU is that you can paint on several layers in quick succession, while one big advantage of resin is that you don't NEED to paint on several layers in quick succession. ;)

Of course this is precisely why Baskin Robbins makes 31 flavors.

Jim

[Slope Nut]

Herb, before you get too far on the wing fillets, let me give you a piece of advice (from experience on this kit). Remove the wing fillet balsa you already put on and throw it all away. Instead, cut a base piece out of 1/64 ply, sandwich it between the wing and the fuse, glue it in, then pack in lite spackle type filler. This is the method Jim uses on his P-47, and it works MUCH better than the BC method. Trust me, your going to be sanding that darn thing until christmas.

Just a quick tip from the man who trashed his spitty on its maiden flight :(

Greg

[Herb]

Well unfortunately that was an old picture, the wing fillets are <A href=http://aeneas.ps.uci.edu/edf/spit/pa230030.jpg> done </A> now - it was indeed quite a bit of sanding :) but not too bad. And I did use a good dose of lightweight filler as well. What I am afraid of is that big blocks of filler can crack & take forever to dry... In any case my fillet balsa was really soft so it worked out ok...

[Gerald]

I tried both methods on my FW190 and decided in favor of the epoxy/glass method. I used the Polycrylic on the horizontal stab and elevator and it immediately warped due to the water absorption. I had to do both sides at the same time to even out the warpage.

The epoxy is actually quite easy to work with. Cleanup is simple because I use disposable brushes and mixing cups. Also the epoxy cleans up with ordinary denatured alcohol which does not have noxious fumes.

I finished the fuselage and wing with .5oz glass cloth and West Systems epoxy as per Jim Ryan's popular method. I also used his idea for wing fillets with 1/64 plywood as a base and lightweight spackle to form the fillet. There was little perceptible shrinkage and no cracking. This stuff is designed to go on fairly thick in one application and weighs almost nothing. You can see it in the photo of my Focke Wolf (http://personal.rdu.bellsouth.net/rdu/g/e/geraldod/FW190.JPG) where I have not yet glassed over the fillets.

The fiberglass/epoxy and primer was quite heavy though adding about 2oz to the 220sqin wing. I think a lot of that was the primer. I used Krylon "supposedly sandable" primer and even after 24 hrs it still rapidly clogs the sandpaper when wetsanding. I used 2 whole sheets of sandpaper to cut the first coat back down to the glass. The second coat was so thin that it's almost translucent and it still seems heavy. Can any of you recommend a good lightweight sandable primer?

[Herb]

Thanks for your comments. I have tried the epoxy method many times, and I agree it works very well. I just don't like the mess but I guess there is no real substitute for it - most of the time.

Of course there is nothing wrong I think in using both methods at the same time, everybody reports the same problem I have had in my F/A-18: warping of tail structures. So epoxy there is certainly the way to go.

The other important comment by Jim Ryan is: water based products can't usually be wet-sanded.

Re the sandable primer issue, I have tried four types (!) recently:

* Rustoleum automotive primer from HomeDepot. Works ok
but has a "greasy" feeling to it

* Pactra Prep sandable primer, the best so far. Goes on thicker
and sands nicely. Pricy at 8$ but superb & seems lighter. Used it on my <A href=http://aeneas.ps.uci.edu/edf/mig15/pa180012.jpg> MiG </A>.

* New Plastikote white sandable primer from an automotive store, seems ok I only tested it, but liked the white color

* New Plastikote Spot Filler & primer, thicker seemed to be cracking on the test surface I used.

My wife told me she thinks I must have been working in an automotive bodyshop in a previous life :) .

[J Bergsmith]

I used the Ryan Method, but I'm having a hard time getting the weave filled. Using Plastikote primer, I spray, sand and the weave is still there. I guess I must be doing something wrong. I used one thinned layer of epoxy on the fiberglass, should I have used another thin layer of epoxy before priming?

[Slope Nut]

I use generic primer from the Autozone auto store. 2 bux a can, and its really good. I too have an extra can of Krylon "sandable" primer. It never dried, even after two weeks! Clogged the paper like crazy.

If you have problems filling the weave, get three cans of the primer you use, and go on light for the first couple of coats, then go on REALLY heavy after that. I put a ton of primer on my plane to get all the weave filled in. I then take it all back off except for that which fills the weave. Works out great, and doesnt add much weight at all (though I have had more practice now!). I used this method most recently on my ME-262, and I am really pleased with the results.

Gerald, how goes the retract installation on that 190?

[Herb]

I don't fill the weave with primer everywhere, I use some lightweight spackle (OneTime from HomeDepot) to fill the weave and apply a light coat(s) diluted with water & with a broad brush. But the Pactra primer seems very close to what I get with the lightweight filler. Also, the lightweight spackle weighs next to nothing. Some automotive primers seem rather <i>heavy</i>...

By the way some parts need to look better than others, after all who is going to look at the perfection of the paintjob <i>under</i> the wing?

The primer is added later to get a really smooth surface & show any surviving imperfections & unevenness...

[Jim Ryan]

Lots of things to reply to! Let's see, where to start....

1. Herb expressed concern about the drying time for lite filler wing fillets, and also wondered if they would crack. I first used this technique on a .70 4-stroke P-51 that I built circa 1994. I allowed about 3 days for drying, because the dryer it is, the better it sands. I then glassed over the fillets with 2 ounce cloth and resin. I sold this model when I got into electric power, and 7+ years later it's still giving faithful service to its new owner. There's not a crack or blister anywhere on the wing fillets.

The glass cloth and epoxy resin is crucial to this technique; it's the glass and resin that imparts the strength, and the filler is just providing the basic shape.

2. Gerald complained about problems with the Krylon "sandable" primer. Join the club! I used to really like Krylon primer, but somewhere along the line they changed the formulation, and it doesn't sand worth a damn. These days I mostly use Plastikote sandable primer purchased at Pep Boys. It's almost as good as commercial acrylic primers, and it has the benefit of being fast and convenient to use. As for your weight gain, you should be sanding off nearly all the primer; the cloth should just have a "dirty" appearance to it because the primer should only be down in the weave. This is why a good sandable primer is crucial; it needs to sand a lot more easily than the cloth/epoxy, or you'll have problems with sanding through the tissue-thin cloth.

One more thing: adding a couple of drops of detergent to the water will reduce the tendancy of the primer to clog the sandpaper and allow you to salvage this wing. BUT, it will also make it easier for the water to saturate the wood. If you do this, work carefully and dry the wing thoroughly every few minutes.

3. John expressed concern about not getting the cloth weave filled. I spray on a couple of fairly heavy and uniform coats and let them dry. If I see a lot of exposed wood grain (oblong pinholes), I squeegee slightly diluted lite spackle over the airframe to pack them full. Then I wet sand off nearly all the primer. If I'm going for a really good finish, I'll then spray a second but much lighter application and wet sand it as well. This should result in a silky smooth surface ready for color coats.


Jim

[Slope Nut]

I do what Jim says :)

Except (always an except), I dont even bother with spackle. If there are holes showing through the first two coats of primer, I just glob it on heavy in those areas to fill it in. It takes longer to sand in the end, but I dont have to go back over it again with more primer after the first sand-down. Besides, as Jim says, nearly all the primer should come back off. You just want the primer down in the weave spaces.

Greg

[Gerald]

Originally posted by Slope Nut
Gerald, how goes the retract installation on that 190?

I believe I've got it all worked out pretty well. Just need a little final tweaking to elimate some friction and binding in the Robart micro retracts.

I'll post more on it and pictures in my thread on the 190

[Eddie P]

>>>>>The other important comment by Jim Ryan is: water based products can't usually be wet-sanded.>>>>>>

I just finished sanding a huge fuselage and 2 big nacelles that were glassed with epoxy finishing resin first. Then the fill coats were WBPU for a faster and easier to sand finish. (The underlying materieal is foam and I've had problems with primer attacking the foam under the glass before - so I use WBPU to "fill" the weave). My experience is that WBPU sands wonderfully using wet sanding techniques, a much faster process than epoxy and primer sanding. It doesn't take off all of the WBPU at all, as lng as you have allowed the coat to fully cure before sanding (overnight). 320 grit used wet, IMHO, is the way to go for this.

When I'm painting glassed foam models I usually skip the primercoat and spray directly onto the prepped and sanded WBPU finished surface. I know it sounds different but it works well for me, and I save the weight of the primer.

It is easier to wet sand WBPU to a perfect glass finish than epoxy (for glassed foam projects), but I still go for Jim Ryan's primer fill technique when glassing balsa surfaces for the overall weight savings and better strength qualities of epoxy used as the resin.

Ed

[Herb]

Ok very good and very interesting. I shouldn't try to put any words in Jim's mouth, I think he pointed out that epoxy resins can be wet sanded within say 4hrs or so of application. What you point out is useful too, namely that if you are willing to wait just a bit more for cure (say overnight) then you can wet sand it too. And of course on/close to foam you can't use anything that is not
water based as it will dissolve the foam...

My question would be: which polyurethane do you use - I suppose they are not all created equal - did you or anybodye else try </i> Varathane</i>.

And secondly did you try by any chance the <i>Polycrylic</i> and how did that wet sand? I haven't tried wet sanding it, but it dry-sands fine...

[Eddie P]

Herb-

I have used the "Dymond" brand found at Home Depot or Ace hardware (etc.) and it was good stuff. I most recently used HSB, a German WBPU made specifically for models. James Frolick (fellow Ezoner) sent me some to use on an EDF almost two years ago and I still had some left that I used on this project.

I have also tried another brand, I think it may have been Thompson's, and it was similar but just a hare more effort to sand. But I don't think I have used Polycrylic per say.

Ed

[Sligh]

I recently finished a pair of Herr floats to go on my SR Cutie using WBP and silkspan.
I used Minwax brand and after attaching the silkspan with the first coat I mixed talc with the second coat to fill all the weave and any pin holes, sanded it, put on a coat of Dupont 30S primer and sanded most off, painted it with an aluminum spray auto paint.
All worked great and added very little weight
.

[Herb]

Originally posted by Eddie P
I most recently used HSB, a German WBPU made specifically for models.

Yes I was very interested in this product as well since I saw it mentioned in European magazines. It seems not to be available here in the US. I posted a question here some time ago on what it consisted of, and got no useful reply at that time...

The Winchester Models <i> HSB finishing system </i> is - as far as I understand - a combination of products. Can you describe the water-based polyurethane that comes with HSB, is it like Dymond Varathane but thinner, is it satin or glossy?

<img src=http://wemotec.com/pictures/spachtel.jpg>

http://wemotec.com/deutsch/zubehoer/glas_lack_motor.html

[VaVol]

Polycrylic by Minwax is lighter than most of the other brands.
I have tested just about all the "name" brands that are sold to the general public.

The gloss type have more filling abilities than the matte type but has more weight. It can be used as a sealer coat and then sanded down.

Use a non clogging type sand paper. I use a white colored carbide paper. use a dust mask!!!

You may use an artists acrylic colors from any craft supplier to tint the Polycrylic and save on finish coats.

Just play around with them to get the shade of color you want.

Wet sanding dose not mean just water. You can use mineral oil in place of water. it will not soak into your air frame as fast as water.Just be sure to clean the surface afterward so the finish will adhere to the coat under it.

Have A Ball

Hans

[Eddie P]

HSB seems thinner than most of the hardware variety and lays down pretty flat after brush marks run together. It is pretty glossy when dry.

It has some subtle differences but all in all it's not a whole lot different than many store bought WBPU's in my opinion. If it was here in the US and on the shelf I'd certainly buy it over other products, but where can we get it in the US?

The only drawback to it is the multiple coats that have to be applied versus 2 thin coats of epoxy and then priming.

Ed

[Herb]

Vavol, I already tried Polycrylic and liked it - I used to glass the bottom of my F/A-18 - it goes on very thin and light - but it seems to take forever to cure to a hard shell! Did you try to take a nice lump of Polycrylic, as I did, and let it dry for weeks in say a cup on your workbench. And then you wait and you wait for it to finally get hard :) ...

Eddie, that's what I suspected, the HSB stuff is thinner than that Varathane floor stuff, the gloss type I tried was too thick & heavy. Maybe I should try the satin version and see if it's any better.

[VaVol]

Herb:
Do you use a foam or hair type brush or air brush to apply the Polycrylic? A foam brush is better for me. I can lay on a very thin coat with one. By the way thin is the way to go. Air brush is even better. Very thin coats.

All water based polys dry by evaporation. Humid air is a killer.
Moving air over the surface speeds the drying. Esters can be added, but this is not suggested as it messes with the adhesion.

I have had good luck with the folks a Minwax about answering questions. You might just call them.

A 75 watt flood lamp also will speed drying.

Hope this helps you. Feel free to ask for more info if needed. I like to give back when I can as Ezone has given so much info to me.

Have A Ball

Hans

[William E Brown]

My experience is to never use anything with a Krylon label, they spent all their money on advertising, instead of developing good products.

[KenP]

Every balsa plane I do now I use WBPU and glass cloth. But I first put on a the balsa a coat or two of oil based PU. Once the oil based PU is dry I give the plane a wet sand with 600 wet/dry, the plane will then have a smooth plastic feel to it. Then I use the cloth and WBPU I get an nice smooth strong finish. On filling the weave sometimes I do it sometimes I don't. On WWII birds I no longer fill the weave, I like the flat finish the unfilled weave gives. But on some foam pusher jets (I only use the WBPU) I fill the weave with the light weight spackle. The spackle seems to give a nice smooth finish, witht the spackle I am using the light weight stuff so I no longer cut it. How much spackle can you pack into the weave of .56oz cloth anyway.

Just my 2 cents.

[jheckmann]

Hi All,

This thread is of interest to me, not building a model, but adding balsa fairings to jury struts on an experimental.

The balsa will be covered with Polyfiber fabric and painted with their process. I would like to make the balsa just a bit harder surfaced and more resistant to damage, prior to fabric covering.

The Minwax water based Fast Drying Polyurethane (clear satin) goes on nicely but two coats are not quite hard enough, after about a week.

Am I using the right stuff? Does it just need another coat or two? Another week to dry?

Thanks for any help....

J

[arx_n_sparx]

J: Welcome to RCGroups! You'll have to pardon me while I turn green with envy.

If hardening the balsa is the only thing you need to do, I'd seriously suggest using CA (cyanoacrylate, or superglue). If you use the thin stuff (it comes in various viscosities) and wipe it on quick, you will have a nice hard surface. Just remember though that it will be a bear to sand afterwards. I think this will give you a nice hard and light surface.

I haven't used the Minwax myself - I did my glassing with Flecto Varathane water based instead. I think it dries faster than the polycrylic.

A couple of links for you to peruse:
http://www.rcgroups.com/forums/showthread.php?t=176211&highlight=cover+foam+with+cloth
http://www.rcgroups.com/forums/showthread.php?s=&threadid=161218

Brad