Polyurethane

[HELModels]

I tried this question over in builder's workshop and didnt get much. What I did was lay up some fiberglass cloth with gorrilla glue/polyurethane in a mold. I then filled the part with spray foam/polyurethane. Now, what I want to do is fix some of the minor pin holes on the exterior and apply some kind of spray paint.

The finish is smooth, so what kind of paint will stick?

If I sand the outside, do my choices increase?

Thanks,

Harry

[chem1]

You might try Krylon but a little light sanding might be required first to take the gloss off. Nice finish, tho. Are you worried about the paint attacking the PU foam?

Mike

[HELModels]

Thanks, actually the foam will be covered by balsa sheet before I paint. I plan to make an internal structure for holding all the gear. The main concern was getting a finish over the PU glue/glass so the sun wont ruin it.

I started sanding tonight and noticed that now I will need a filler. I am going to try PU glue and microballoons. The microballoons might help the sanding.

Thanks,

Harry

[HELModels]

Who'd a thunk that microballoons and polyurethane glue would make a good combination? As I sanded the molded PU, I noticed there were either airbubbles or pinholes in spots that went pretty deep, so I mixed up PU glue and microballoons and made a slurry to fill. It is still drying, but it didnt run all over the place like PU glue will and it seemed to kick off much much quicker than PU glue alone.

I'm going to finish this one, but I've got a better idea how to control the finish on the next one.

Harry

[J Morgan]

Use an auto laquer sandable primer, let dry, wet sand, and then you can use acrylic or Krylon for a nice finish.

J

[HELModels]

Use an auto laquer sandable primer, let dry, wet sand, and then you can use acrylic or Krylon for a nice finish.

J

I've been sanding the microballoon mix and it works but breaking the skin is the hard part. Once I do that, then the sanding isnt so bad. I used an xacto saw blade as a rasp by dragging it backwards away from the teeth. Drag it the other way and it digs in too much. Then I sand with 220 and then 400. I also keep weighing to see if I gained anything and so far not enough gain to register the next 1/10 of an ounce.

When I paint, it will be primer, wet sand, paint.

Thanks,

Harry

[HELModels]

No, No, No, dont wait for the PU and balloons to fully dry. That is what I found out tonight. When the fill mix got to the point I could press it in by finger, which was about 1 hour, and not have it stick and it was springy to the touch, I started sanding and it behaved exactly like epoxy and microballoons. It was so much easier to sand and blended in to the surrounding areas perfectly.

Harry

[HELModels]

Here are 2 pictures of the filler process.

[coloradoz]

Do you have a reference for how you laid up that mold with gorrilla glue/polyurethane ?

[HELModels]

There is no thread on it or web reference, as far as I know. I am pretty sure I came up with it on my own, as noone would be foolish enough to try it. ;)

I used the same techniques as epoxy and glass, just substituted with gorrilla glue. I made sure I blotted up the excess with paper towels and avoided puddles and uneven amounts. I hovered over the mold the whole time and when it got tacky like it was hardening, I burnished the glass into the polyurethane/Gorilla glue. This was the key, and it also gets rid of most air bubbles. My mold has some corners and vertical sections which made it tough, but a very curvy mold should be easier to control.

As you can see, I sanded the result to prep for paint when I might have been able to prime over it, just dont know. I'm at least getting rid of some of the minor blemishes even if it is the hard way.

If you try it or just do a test section, go ahead and post your results in here.
Make sure you wear gloves.

Harry

[coloradoz]

So, I'm understanding that you used Gorilla glue right out of the bottle - not thinned with anything - no exposure to water or anything like that - right? And it hardened? Man, have a tough time getting that stuff to cure when there is no moisture (I live at 8000' and its pretty dry up here).

[HELModels]

I use the relative humidity where I live in MD and it has been very dry lately. I dont know if it will work for you or not. Maybe you could boil some water in a tea kettle and waft the steam over the laid up mold. But, yeah, that is how I have been doing it - straight out the bottle. Wet the cloth out fully, then blot. For the microballoon mix, there is something about the balloons that speeds things up.

Harry

[HELModels]

I've been sidetracked lately with other stuff and trying to figure out how to mold a wing. I made a wing saddle and nylon bolt mount, just need to make a wing mold.

My last batch of microballoons and PU, I went heavy on the microballoons and I've concluded the balloons cause the PU to foam quickly. I'm guessing it causes more moisture laden air to contact the PU. The foaming isnt like the usual PU foaming, though, and is more consistent. It forms a skin quicker and makes a dense foam. I'm not so sure it is that useful, but if you are stuck and only have microballoons and some polyurethane glue and need a filler, this would do.

Harry

[HELModels]

o.k., this polyurethane and microballoons has me thinking about different versions of polyurethane. There is WBPU and silkspan, and there is baby powder, as found here:

http://www.rcgroups.com/forums/showthread.php?t=728586

There are alot of possibilities without using epoxy and I hope to explore them here. Epoxy is expensive and a pain to mix. I'm actually considering 77 some cloth covering into the mold and then polyurethane. The PU glues are strong as all and resistant to cracking. If I take some profiles from the mold, then I could build a balsa/ply structure for the strength.

[HELModels]

I started another layup tonight, using Elmer's Ultimate and Microballoons. This was different than the first layup from the mold. The microballoons kicked off the foaming process almost as soon as it is mixed. I used enough glue to just cover the bottom of a plastic cup and then added the balloons until I had about 10 times the original volume. I smeared this into the mold to get complete coverage. I then smeared out the excess without exposing bare mold. I waited until I started to see some expansion and then plopped the cloth in, pressing it into the mix. It took about an hour and a half from the time I started cutting cloth and mixing glue until the layup was dry to the touch. I still "burnished" the cloth at the point when the glue was just tacky.

The picture shows I might have a dry area but overall it went smoother this time, easier. This will be my cowl after I cut some areas out of the full fuselage to lighten and compensate for the cowl's weight.

Harry

[HELModels]

The good news is that the cowl halves came out nice. The bad news is that I opened the mold too soon after joining and they seperated. This wasnt entirely bad, since the cowl would not join and go around the fuselage. This problem has been solved by some finagling, but requires more finishing work. The cowl fits very nicely now, just needs sanding and paint.

The sanding is needed due to how I solved the fit problem and not the finish of the PU and balloons.

To repeat the good news, I laid up the cowl using microballoons and polyurethane glue and fiberglass cloth. It works and the finish was very smooth. The microballoons lighten the glue, give a more uniform finish, all without sacrificing strength. I am relying on the cloth for the strength and PU for the finish and flexibility.

Harry

[HELModels]

Here is a picture which shows the cowl. You can see it is whiter than the rest of the pod. This is because, I am using more microballoons than when I started this process. The cowl needed alot of work to fit the pod and the nice polyurethane finish was ruined, so I have been filling and sanding.

Right now, the empty airframe weighs 8 oz. I dont know if that is heavy for a 42 inch span plane and I dont know the wing area yet. It should allow me to use a 750 mAh 2s. The stock aerobird challenger weighs in at 17 oz. RTF, and I have been fiddling with this wing for awhile on earlier version of my design, so I actually do know that it flies great at 12 oz. I wanted to be under 10 oz. final weight, but that will require another attempt.

Harry

[HELModels]

I've done some more experimenting with the polyurethane glue and glass, but it involves paint. I was looking for a good paint to go over the sanded finish. I chose Rustoleum for Plastic. I wanted to be able to change the color so I put packing tape over the surface first, so actually this is an experiment in paint over plastic coverings. :D ;) . I tried it tonight and it adheres perfectly to packing tape. It should do equally well on mylar or heat shrink coverings.

It dried within an hour, as the instructions said it would. I only used the packing tape on the cowl and did actually apply the paint to the rear pod 1/2, so I do know how it applies to PU and glass. As I applied it I noticed I could see the cloth weave coming out. I needed to apply some spackle, or some PU and balloons. Overall, I am satisfied with the finish and know how to get it right the next time. If I go for the smoothest finish on this one, I will need to sand this down and fill some more, then paint again. This paint specifies that another coat should be applied within a few minutes or after 48 hours.

Harry

[HELModels]

Here are some pictures. I like the shape, but the finish and fit needs improvement. You can see in the picture that there are some gouges in the rear of the pod. I had done some surgery on the boom and needed access to get some glue in there. I didnt bother to fill it, knowing the next one is on the way. The cowl has some gaps visible from the rear. This will be improved by using the mold properly. You can also see where belly landing on the cowl has scraped the Rustoleum for Plastic off the packing tape. The next one wont have that.

Overall, I am pleased with using the polyurethane and glass. It is extremely strong and cheap for the amount needed on something small. The process isnt totally forgiving, but I really believe I can make it work. You get a real smooth finish initially, but the color is awful which is why I sanded and filled and then painted.

It does look great in the air with the fat and curvy pod now painted white. The Rustoleum for Plastic has no runs and went on real nice over the PU/glass. The only blemishes are due to a lack of filling and sanding. It seems real durable on a good surface with no signs of peeling or cracking. It will come off packing tape if scraped with some force, like belly landing on gravel. The rear pod shows no signs of wear from that.

Tonight, I prepped the mold for the next try, which wont require any sanding.

Harry

[HELModels]

In the interest of anti thread pollution, this thread will end up with a painted finish. If I learned nothing watching my father as a kid, it is that preparation is 90% of the work towards a good finish.

So, what did I do tonight? I thought ahead to the next attempt and how to get that nice finish. I already had smeared some pva into the mold the other night. Tonight, I built a joggle gasket using chavant clay. This clay comes with a fiberglass kit from cstsales.com which I bought years ago. I cut off a chunk and heated it in the microwave with a few drops of water, wrapped in saran wrap. Then I kneaded it until my knuckles cracked and then heated it some more until it was very pliable. I rolled it into a rope shape and put it around the rim of the mold. I then trimmed that flush with a razor blade and pressed it some more around the mold. Then I gently pushed it some more until it started to move beyond the edge of the mold, into the interior. I worked it carefully, so that the overlap would be uniform and only about a 1/16 of an inch. This will leave enough room to join the halves by putting the adhesive of choice into the joggle seam.

I also applied pva to the chavant when I finished. Here is a picture.

[HELModels]

Here is the progress toward the ultimate goal of a nicely finished model. Tonight it was put some paint down in the mold, after a few coats of pva. The pva also came with the fiberglass kit from CST, but is available at boat stores.

The joggle gasket will get some more pva over the paint, so that the chavant clay wont stick to the layup. It will do this even with pva, but the paint will act as an additional barrier, so I can just peel the gasket away when ready.

Here is the picture.

[HELModels]

I painted the molds, then layed up the black 1/2. I realized the white half needed another coat and needed to wait 48 hours since I had not applied the extra coat within an hour. So, what happened to the black 1/2? I pulled it from the mold to see what kind of bond I got between the paint and PU/glass. I saw wrinkles in the paint and air bubbles. This had nothing to do with the paint type. The wrinkles were where there was no bond. I reapplied the pva and then painted again.

The next layup attempt will involve a light scuffing of the paint and a smeared on coat of PU, first. The next layer will be the PU/glass which will bond much better, more uniformly to the smeared on partially cured layer of PU, than to the paint directly. The problem is as a thicker layer of PU/glass starts to cure, it must be burnished evenly and at the right moment. If the bond to the paint is a thin layer of PU, then the PU/glass layer can have some small bubbles and the paint layer will still have a solid backing. It is made for plastic and is flexible, and will hold it's surface better if it has a rigid and uniform skin of PU.

I have to admit that doing this with epoxy resin and glass would be much easier, but the convenience of the PU is worth the effort, once the technique needed is mastered.

With the black half, I did find that a sheet of cling wrap saved me skin contact with the glue. I pressed the cling wrap into the layup and was able to squeeze the excess to the edges of the mold. When the PU cured, the cling wrap came right off.

Results say it all and I'll post them when I can.

Harry

[HELModels]

O.K., I pulled the cowl from the mold today. It aint perfect, but it has paint, is shiny, and the halves are joined. I screwed up the parting line which has something to do with the joggle gasket - the paint didnt get under the lip and you can see a gap in the paint. I also didnt wait long enough for the black to dry before applying the PU skin. The white side is better because it sat longer before I applied the skin and layup, and I used more paint on that side. Lots of little problems which should get easier to avoid with another try.

The basic steps:

1. wax
2. pva
3. paint - dont skimp, lay it on thick and let it dry
4. very, very thin coat of Pu over the paint - let it dry
5. PU/microballoons & Glass layup - put saran wrap/cling wrap over and press out the excess
6. sand the inside when dry to lighten it - optional
7. join the halves - wait 24 hours before pulling


Here is the picture:

[gzkpez]

Interesting, is it stiff?

[HELModels]

Greg,
Absolutely strong as a gorilla. ;) I think one of the keys to what success I've had with it is that I started mixing it with microballoons to a mayonnaise consistency. It kicks off quicker and the foaming is more uniform, more closed cell appearance, seemingly less density. The cowl in the picture above is stiff, yet has some flexibility.

Harry

whoops, sorry, dont know where I got gary from. A quick search showed you use the name greg. If I'm going to refer to you by name, I better get it right.

[HELModels]

I'm working on attempt #2.5 right now. To get the cowl to fit right, I need to compensate for the thickness of the layup. To do that, I layed up another cowl. I squeegeed up the excess before I put it into the mold. This worked nicely and was a lighter layup. When it dried/cured, I sanded it as smooth as I could with it still in the mold. Then, I ironed on some solite covering. This smoothed it out some more. I applied PVA.

The whole point of doing this is that now when I lay up the whole pod(mold has a back half which attaches to the front), the pod will have a step where the cowl attaches, it will fit. It's great to have a mold, but you have to learn to use it, even if you are using polyurethane in place of epoxy resin.

Pictures will follow. The next attempt should have a painted in the mold finish with a cowl that slides right over the pod. That was the goal and this is the place for finishes. Using a mold is just one more method of getting a nice finish on a model.

Harry

[squidbait]

Harry,

Very cool process you're working on here, I'm looking forward to seeing your next results.

What're you using to burnish the glass cloth into the glue?

[HELModels]

Heh squidbait,
To burnish, I wait until it is starting to harden but still pliable and then press and rub with my fingers. I do it a few times until it is dry. If the cling wrap is over the cloth, then there is no skin contact. I could probably use a better tool for that.

[HELModels]

Here is the next attempt in progress, waiting for the paint to dry.

[Patrick Plawner]

very nice !

[HELModels]

Thanks, Patrick.

I laid up the halves tonight and I should be ready to join them tomorrow. I havent found it a problem to apply PU over PU after it has cured. That isnt the method of choice with epoxy - you get a better bond when things are still green.

The parting line is the question mark with the latest layup. The last attempt had some paint peel off near the join line. To avoid that, I oversprayed onto the surrounding mold face. When I trim the fiberglass, I will trim flush with the overspray, then join the halves. When the halves are ready to come out of the mold, the overspray will come off with the part, and then I can trim it with a razor blade.

We'll have to see when it is ready.

[HELModels]

man-o-man, It almost worked. I didnt get enough wax and/or PVA into one of the corners and along one edge. When I pulled the joined part, some paint stayed in the mold. This is a problem whether using epoxy or this whacky polyurethane, so no matter what I'm getting practice in the painting in the mold process.

Things that worked:

1. Wax where I used it enough
2. PVA, same as above
3. Barrier coat of polyurethane smeared over the paint
4. Overspraying paint onto the mold joining faces
5. Applying microballoons and PU to the cloth and spreading it with a credit card
6. Use #5 and burnishing isnt necessary

[HELModels]

This is the picture of the latest attempt to get a painted in the mold finish over polyurethane and fiberglass. You can see the edge under the wing where the paint stayed in the mold. There are also several much smaller areas. The cowl had it's own set of problems which were solved by using the barrier coat of PU over the paint. The parting line had some problems too, which were solved by overspraying. The rear part of the pod is the latest attempt. The parting line is better and it also employs the barrier coat to prevent air bubbles under the paint causing softer depressions.

The bottom line is that it will take practice and familiarity with the pitfalls, whether using PU or epoxy.

Harry