Hi all,

This is my first time using PVA and it did not come out as I had expected, planned, or desired. So I'm looking for advice on technique or alternatives.

Here is the problem in detail: I have a set of half-plugs (left and right fuselage sides) mounted to a backer-board and inside a frame. The plugs have been painstakingly finished and primed and sanded all the way down to 1200-grit sandpaper followed by #0-0-0-0 steel wool to buff the primer surface up to a nice shine. I followed this with 5 coats of wax, buffing, and polishing between coats. Everything was nice and shinny

Now comes the step that failed. I brushed on a thin coat of PVA and let it dry, then over an hour later added a second coat of PVA to the plug and the board. I let the whole assembly dry for 4 hours. Since this is my first time with PVA I also ran a test piece of flat wood side-by-side of the plugs. The flat piece looks better but the gracefully rounded and sanded plugs show visible brush-strokes and running my hand over the plugs I can feel pin-prick sized high points and an occasional drip.. I know if I went from here to applying the tooling resin, all of these unwanted features would be perfectly transfered to the tool.

So I peeled off a piece of the offending PVA and applied another coat but have the same results. From what I've read, the PVA should flow smooth as it dries and that brushing it own it the way to go -- it seems to work fine on flat surfaces.

Ok, so I'm asking the experts for some advice on how to get past this PVA issue and get on to building my tool -- I have three to complete.

Thanks in advance,
John

I spray it with a gravity feed spray gun when I want the best finish.

I spray with 35 psi. You can thin it with 10% water if you are having problems with it not flowing flat or drying too fast. When you spray it onto the surface it will first look milky. As it sits all the little bubbles will pass out.

The biggest problem is dust control. Any little spec will show up in the finish.

If you don't have spray equipment then the key to brushing is:

1) I use a foam brush
2) Use light pressure
3) Keep the tip of the brush very wet with PVA. Dip into fresh PVA often
4) Try to apply a flowing coat without the need to go back and rebrush where you have already been.
5) Make sure that the surface, brush, and PVA is dust free.

Adam

I have been down the same road. Brushing on the green PVA works for basic layup's but if you are going for the super smooth Class "A" finish with no runs, drips or errors you are going to need to go to spraying it on. The first coat has to be a light mist coat and then more on the second coat. Let it set up 5 to 10 minutes and then you can lay up that heavy barrier coat.

I've never had any luck brushing it on. Maybe some others will jump in with ideas. There are other systems out there which may work for you like the fre-coat system.
Steve

I've started using a foam brush too, and found it better than a bristle brush. Still end up with some ripples, runs etc. but I just live with it as I don't have a spray setup. Thinning it seems to work too, though I go a little higher than Adam at ~10-20% water.

I've also started doing just wax only in a moderately seasoned mold (after about 6-8 parts) that has been painted. The 6 or so I've done like this seem to release better than with PVA, but that might just be my memory and the finish looks better too.

Steve

Thanks guys for the quick responses!

I'll try the foam brush route tomorrow (after I get some foam brushes and get the existing PVA off of the plugs) and thinning the PVA with some water. I'm probably "brushing dry" without realizing it while I try to avoid drips.

The "spray" approach looks like the right way to go later on but now I'm trying to keep the "things I am doing for the first time" down to just shy of "everything" ;-P

Since these molds with be the masters for any parts I pull later on, I'd like them to be as close to perfect as I can get.

Thanks,
John

Getting a "perfect" finish from PVA is a 100:1 shot in a home shop. Dust is the issue.
Spray on is best but takes practice.

Wax the snot out of it over 2 days and do the lay up.

Originally Posted by wyowindworks

I spray it with a gravity feed spray gun when I want the best finish...
Adam

Adam,

Is this the type of gravity feed air gun you are talking about?
http://www.harborfreight.com/air-too...gun-47016.html

or more like this one? http://www.harborfreight.com/air-too...gun-66222.html

It doesn't look that hard to add to my list of things to learn. For cleaning, I assume that you just run a lot of warm water through the gun (so that it is not a one-use-only tool)?

Thanks,
John

Originally Posted by F3X

Getting a "perfect" finish from PVA is a 100:1 shot in a home shop. Dust is the issue.
Spray on is best but takes practice.

Wax the snot out of it over 2 days and do the lay up.

Since I'm making the master mold here rather than parts from an existing mold, are you suggestion that I can skip the PVA step and just wax-on wax-off 10-15 times?

Thanks,
John

John,

Be sure to get the Harbor Freight gun with the 1.4mm tip, it will be useful for PVA and for 2K clear coats. For 2k color you might want a smaller tip, but that's off topic.

After spraying PVA, clean the gun with warm water. Learning to use an HVLP gun is easy, just takes some practice and lots of fooling around with the fluid control, the fan pattern control and the pressure adjustment.

-Alex

Originally Posted by Ne-Aero

Since I'm making the master mold here rather than parts from an existing mold, are you suggestion that I can skip the PVA step and just wax-on wax-off 10-15 times?

Thanks,
John

Yep,

But let me ask what primer did you use?
What are the plugs made from?
Any Bondo showing?
What resin are you using?

If you used a catalyzed polyester primer and have 100% coverage than you are probably OK. If you have bondo showing you may have issues even if its as smooth as possible. If you used any primer from a spray can it will probably stick.
I typically use 1 piece plugs and splitter with alignment pins for a perfect mate. I am not a fan of 2 piece plugs. hard to get a good mate. Join in the mold where ever possible.
You want to use a finish coat that is catalyzed. Duratec finishing primer is about the minimum. I prefer painting and polishing. Again with a catalyzed paint. Polyester gelcoat works but can be a lot of sanding and polishing. A 2 part urethane is easier.

If you think you may have a potential issue by not using catalyzed products than you will need the PVA to be safe.

Good luck with the project.

Originally Posted by F3X

Getting a "perfect" finish from PVA is a 100:1 shot in a home shop. Dust is the issue.
Spray on is best but takes practice.

Wax the snot out of it over 2 days and do the lay up.

I agree. This is why I use Frekote to make all my molds. You must use the sealer and the plug paint must be a 2-part catalyzed paint to work with Frekote. The plug releases are simply amazing. Simply wipe on wipe off. Perfect surface duplication without the dust issues.

Originally Posted by Ne-Aero

Adam,

Is this the type of gravity feed air gun you are talking about?
http://www.harborfreight.com/air-too...gun-47016.html

or more like this one? http://www.harborfreight.com/air-too...gun-66222.html

It doesn't look that hard to add to my list of things to learn. For cleaning, I assume that you just run a lot of warm water through the gun (so that it is not a one-use-only tool)?

The first one would work well. My favorite harbor freight gun was one just like that but had a 1.4 mm tip rather than a 1.5 mm. The 1.5 mm will work but I like a 1.4 mm better. Unfortunately they haven't had the 1.4 mm 20 ounce gun on their site for several months.

You can use water to clean up your gun.

Originally Posted by Ne-Aero

Since I'm making the master mold here rather than parts from an existing mold, are you suggestion that I can skip the PVA step and just wax-on wax-off 10-15 times?

Maybe. Wax is UNABLE to block the potential chemical reactivity between the paint and the resin. You can do it, but you should do a lot of testing. The internet is loaded with failures of people who used just wax. Many blame the waxing process but it's really a reactivity issue. If the paint is reactive with the resin then 1000 coats of wax is unable to block the reactivity. Doing a bunch of coats is also a waste of time. The first coat of wax has the highest build up. Each following coat does very little. The manufactures recommend multiple coats to insure complete coverage (4 to 6 coats). I've talked with the chemists at Meguiar's and Rexco. They both say the same thing. They both said that doing a lot of coats is a waste of time (more than 6). The carriers within the wax soften the previous coats resulting in a layer that is no thicker.

Typically 2 component catalyzed urethanes work best when using just wax. I've still seen epic failures with just wax and auto paints. Also, you want to make sure that your paint is fully cured. If the paint isn't fully cured it's reactivity will be heightened.

The job of the wax is to provide slip. The role of the PVA is to create a barrier that blocks reactivity.

BTW, you can't use just PVA. PVA needs to be applied on top of a waxed surface.

All good information from you guys. I did spend a couple of months going through old threads here looking for tips and failures etc. before deciding to go down this path. Samotage and his JART build was a fun one.

My process:

• The plugs were caved from three pieces of 3/4" MDF glued together and shaped.
• Once shaped bug voids and nick's etc were filled with Durham's Rock Hard putty and resanded to shape.
• Then a heavy coat of West Systems Epoxy with a light sanding before the second coat of epoxy (MDF sucks up epoxy fast).
• That sealed the plugs.
• Then there were mulitple iterations of Krylon Industrial Tough Coat primer from a spray can.
• Multiple sandings all the way up to 1200-grit followed by #0-0-0-0 steel wool to buff the plugs back to a shine.
• The plugs are screwed to a backer board which gets a 3" wooden frame around it to support the mold/tool.
• 2 coats of wax then the PVA -- which is where I stopped.
• Once I get the PVA set correctly another coat of wax goes on
• then the tooling resin
• then 10 ounce glass cloth and finishing resin.

The thread I'm covering this on is here:: http://www.rcgroups.com/forums/showthread.php?t=1421281

Ok, so tomorrow I get a new toy HVLP spray gun and go play with spraying PVA and seeing how that goes. The nice thing about the PVA is that I can either peel it off or wipe it off with a damp cloth.

Thanks to all.

John

Wax on top of PVA will be a nightmare. When you go to buff the wax, on top of the PVA, you will just tear the PVA right off the plug.

1) 4 coats of wax with 15-30 between applications
2) Let your last application of wax sit for 1 hour after buffing before applying PVA
3) Apply the PVA and let it fully dry (1 to 4 hours depending on temperature, humidity, and thickness)
4) Apply the the surface coat and let it get tacky to the touch
5) Start with light glass (1 ounce), then 2 ounce, then tight weave 3 or 4 ounce, then some heavier tooling fabrics. If you go to the tooling fabrics right away you will get the fabric pattern telegraphing all the way to the mold surface.

Originally Posted by Ne-Aero

All good information from you guys.[*]Then there were mulitple iterations of Krylon Industrial Tough Coat primer from a spray can. [*]Thanks to all.

John

Better use PVA then. Make a small spray booth from plastic sheet. Spray PVA using the directions on the bottle and let it sit in the booth to cure. If you have to re apply the PVA then put down a coat of new wax and let it cure after the plug is cleaned. Don't spray directly over the cleaned up plug.

What wax are you using?

Partall #2 wax works well with PVA, Its a bit of a pain to polish out except when its fresh. Meguiars #8 is good also. There are others. Johnson's Past wax works.

I use brushed on PVA over Partall #2 on my corian molds and never, ever have sticking problems. I'm doing wings, tails, and fuselages now and am getting quality better than I ever expected. I can't use many of the semi-permanent releases because of the solvents in them potentially harming my mold material.

R&G sells a very tiny cell sponge for PVA application. I have a bunch of them if you'd like to try them. The finish is absolutely mirror-like with this method, but it doesn't take to tight places well.

I've had no complaints of surface quality at all. I'm not painting (DLG) FWIW.