PVA issues

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#1

Hey gang. I realize this isn’t technically a composites question, but I’m having issues with PVA.

I am molding my fenders to make some custom fiberglass versions…it will be a 4 part mold due to the multiple negative reflections:

And here is how the PVA looks:

I am spraying with a compressor and gun. The psi I’m spraying it is between 60-100 (it’s actually a tiny compressor for a nail gun and the pressure drops off pretty rapidly…but I stop before it gets below 60 psi).

Last time I applied the PVA, I sprayed 10 coats (30 minutes between coats at ~60 degrees Fahrenheit)…but I noticed that it never turned into a film even after 3 days…it just stayed wet.:confused:

My second question, do you keep PVA in your gun between coats? Or do you break your gun down and clean it between every coat? That would get REALLY tedious…but I’m noticing grit in the plastic filter in my gun and this grit is showing up in the surface of the PVA I’m spraying.

Thanks in advance.

#2

Here is a recent thread with tips about spraying PVA.

10 coats is way more than necessary. I often just use 1 flowing coat.

If you are hosing all those coats on a not letting it dry between them then it would take a long time to dry. 60*F is also on the cool side. It will dry much faster if the temps are warm.

When I used to do a lot of PVA work I would leave the PVA in the gun for whole day and then clean it up with water at the end of the day.

If the grit is in the PVA then you need to filter it before putting it in the gun. You should also have an air filter on the gun if your air supply has junk in it.

#3

Hey B, welcome to the forum. I don’t post much here but like to check every now and then, it has an abundance of experience.
Is this for the Honda?
If I spray PVA, I do it in very light coats, not like paint, just light coats. Of course here in the AZ heat is usually dries fast. You could use a heat gun between coats to make sure its dry. If you waxed good it shouldn’t take much to help the mold pop easily.

#4

Holy PVA! I do about ten coats when I’m making a new mold, (If I use PVA) but I spray light mist coats. I never spray full coats.

Buddy, let that sit for a few days and put it in the sun when ever you can or under heat lamps. I hope it doesnt solvant pop on you since its so thick.

#5

It sounds and looks like you are just putting too much pva on. Each coat is too thick. Mist coats is what is needed. Let the coat flash off before putting another coat on.

#6

Remember also that when you build up your PVA lke that, you’re adding (a lot) to the shape of the part underneath and degrading any details like body lines and so on.

You may think about building a simple low temp curing box. You can use a space heater to blow into the box. You have to account for safety here… I will set the (fan over resistive element type) space heater outside of the box so that it’s always pulling in cool air and not over heating. I usually keep it on the low setting which is about 750 watts. I monitor part temps inside and can easily conrol the temp and maintain under 120*F. You can regulate by opening up relief holes in the box. Just make sure you have enough airflow to keep the space heater from overheating. Better yet always use this thing out doors…lol You don’t need a perfectly sealed box setup and can use about anything to make it. Regular tape will hold it together nicely. This setup will save the day. If you try to always keep your working temps the same all year long your results will be as predictable.

If you build a large enough box you can just bring it to your work as needed, set up for a few minutes between coats, pull the box back away and be good to go. This goes for curing your resins and such as well. We don’t have to worry about it to much down here but I don’t like to wait when it get’s “cooler” here in the winter.

I keep a small PVA gun and usually never empty it. As long as the lid is on, the PVA will keep well especially if you use it often enough. I end up cleaning it out every once in a while which is fairly easy.

You can clean that mess back off of there by gently wiping the surface down with a wet or damp towel (Maybe several.) When your finished, let it dry completely and re-spray a proper coat.

#7

Instead of using a heater, consider building an oven from some EPS foam, or even a cardboard box, and some light bulbs.

#8

I’ve found that the tip in spraying pva is many light mist coats, this way it dries faster, also it helps adding water in the pva, it makes it flow better.

#9

In my experience you need to do a flowing coat if you want a flat glossy finish rather than mist coats. Below are two photos of how PVA can look if you shoot it with a flowing coat. Neither of these were reduced and both done in 1 shot out of the gun.

#10

First, thank you everyone for the warm welcome and helpful replies. Yes Jim, this is for the Honda…

Well, as embarrassing as this is to admit, I realized yesterday that this wasn’t PVA I was spraying…it was catalyst for a silicone mold set I bought:facepalm:. I guess I grabbed the look-a-like bottle in haste:(

I realized this after brushing on gel coat:

Here you can see the gelcoat is pulling away from the surface…I had to mix up another batch and brush it on to reinforce the thin areas. I need a proper compressor to spray this on…I’m using a nail gun compressor I picked up at Loews a year ago and it only has a 1 gallon tank:
[IMG]http://i736.photobucket.com/albums/xx10/swolehead/P1050376.jpg[/IMG

I waited till the gelcoat was tacky but would not leave residue on my finger and then applied 3/4 oz csm. Unfortunately this trial run was a major failure…

I think there were 2 major problems, 1) the gel coat needed to cure for a couple more hours (maybe even overnight) and 2) I tried to put too many layers of chop strand mat down at once…I layed down as much as 3 overlapping layers at once in some parts. Also, the fiberglass and the underlying gelcoat popped off the underlying “silicone catalyst”…I gues this is what is meant by solvent pop?

Here are some pics of the failure:

The two ends (by the headlight and down along the lower wheel well) came out really well, but in the middle there was just too much mass of csm, air got trapped under neath, and I couldn’t roll it out. What you see here is after I got frustrated and tried to strip off some of the excess csm:

I will try again in a week or so, but I’m pretty burnt out on this…I spent my whole weekend trying to set up this mold. I will need to buy a bigger compressor and more gel coat before I try again.:wtc:

#11

That’s a’lotta PVA - drying it will take time…if it were me, I would strip that PVA off the part and start over, although more work, it may be faster than waiting for it to dry.

#12

Thanks…again, and embarassingly, I accidentally sprayed silicone catalyst (mistaking it for PVA…same color, same size bottle, same consistency) instead of PVA. I have stripped the fender down and will keep trying until I get it right. Thanks for all the advice.

#13

Well, that would explain why it wouldn’t dry. :D:D

#14

Rebuild your flange, apply 10 mist coats of PVA, brush on your gel coat, layup 8 layers of 1.5oz strand matt :).

#15

Doesn’t this create pin holes? Usually when I try to flow coat I get a specly finish from the trapped air in the flow coat :(.

#16

It depends on the wax. If you use Part-all Paste #2 and Part-all Film #10 (PVA) you can typically shot one flowing/wet coat and be done…at least I used to do it all the time.

If you are using another wax, like Mequiar’s #8, the PVA will bead up and fish-eye. The solution is to spray a light mist coat and let it fully dry. Then shot and slightly heavier coat and let it fully dry. Then you can shoot a flowing coat. If you are getting trapped air from spraying then you are spraying in too warm of temperatures and the PVA is drying too fast. You can either spray in cooler temperatures, increase the humidity by misting the room with a water bottle or use your paint gun with just water, or add 10% water to the PVA.

If your room is really dusty you well get a speckled finish. I often mist the room with water to pull the dust out of the air.

I can make PVA look as good as any clear-coat out of the gun. I’m not boasting…just saying that its is possible.

#17

I always (not in the video) spray the PVA. It is important to use a grounding wax for PVA, than you cam spray one flowing coat.
It needs some practice but it works perfect if you have a dust free room.

#18

I’ll give that a try. I always thought about reducing PVA but wasn’t sure if adding water wold destroy the PVA.

I’ll give it a try :o

#19

The downside to thinning is that: 1) The barrier will be dimensionally thinner, and 2) The shrinkage will increase. It’s possible to thin it to the point that the PVA will shrink so much that it will pull itself out of corners.

I, personally, consider thinning with water to be a last resort.

#20

So I’ll first try spraying in a climate controlled room and see hwo that works.

How do you feel about me misting a little water over the PVA after spraying a full coat?

I’ll prob just have to do some tests…