Preval Sprayer for Gelcoat

Just got a Preval Spray Gun, a neat little $6.99 disposable sprayer for applying a finish layer of gelcoat.

Anybody know how much to thin gelcoat for application with this little toy? What do you thin it with?


Usually you thin polyester gelcoat with styrene. I haven’t tried spraying it though. I’m not sure how much you need to thin it. Probably require some trial and error. I wonder if you’ll be able to get much film thickness that way, or whether it’ll just be to light once you thin it enough to go through the sprayer. I don’t know. Give it a try and let me know because I’ve wondered that in the past.

I’ve gotten good results by brushing on the gelcoat, and using the sprayer to apply PVA over the gelcoat for a smooth finish. That’s another way you could go.

stands for…?

Thanks for the feedback - I’ll definitely post the results of my experimentation.

Some info here…


– Last Updated: Nov-28-09 10:17 PM EST –

and to spray Gel coat, maximum thinning with Acetone is 10%

Best Wishes

Wear a Mask with a good organic filter when spraying or use one for remote air

PVA is an air-barrier, and is only used on top of UNwaxed gelcoat (laminating gelcoat). Waxed gelcoat (or finishing gelcoat) requires no air-barrier such as PVA.

I’m skeptical…
…that a Preval sprayer is going to work with gelcoat at all, but give it a shot and let us know how it goes.

Site says it sprays epoxy and gel coat under “marine uses,” but it looks pretty anemic to me. If it were me I’d call them to ask about how much thinning you really need. I mean, I can get a 99 cent spray bottle to spray epoxy if I thin it enough, but it won’t do me much good.

Prevals work great…
Jacob could respond better with technical advice. We thin the gelcoat with Acetone. We’ve used prevail for gelcoat repair work, not for large scale sprays. I’ll make Jacob aware of this post tomorrow and I’m sure he’ll respond.


Thin with MEK

– Last Updated: Nov-29-09 10:51 AM EST –

Thinning with acetone radically extends gel time.

Adding more than ~3% styrene significantly weakens the gel coat.

Using MEK, a high end paint thinner available in paint shops does not have such adverse effects.

Important to note the difference between MEK the thinner and MEKP the catalyst!

In respraying gel, surface preparation is more than half the battle. Sand with 220 grit, wash very, very, clean with acetone.

It is also worth noting that applying insufficient gel thickness will impede catalyzation. Use a mil gauge; aim for 10 mil.

want it to harden. The prevail works great for spot repairs of gelcoat. I would not use it for more than 1-2 sg.ft. Preheat your hull by putting a trouble light inside the boat for at least and hour before you spray and keep it in there for at least an hour after you spray. I have had bad results with acetone and would try to spray without thining if possible. The higher your temperatures the better the performance, however, the mix will kick faster in higher temps (which I prefer). Prevail is the preferred method for spot repairs in my area. Good luck. Bill

p.s. I would like to hear more about styrene as acetone is no friend of mine.

Skin & Lung Protection
Is MEK more hazardous than acetone? Any additional precautions I should take?

I don’t have the wherewithal for a suit with an independent air supply, but my workshop has doors that open at both ends, good air flow-though and an exhaust fan 3’ away from the boat. I use a respirator with dust/fumes/mists filters when working with solvents, and disposable latex gloves.

Thank y’all for good feedback.

Both are hazardous, but MEK is worse
Good ventilation and an organic vapor respirator should be enough protection.

MEK is yucky, but
MEKP is by far the worst one in that cocktail. Be extremely careful when handling and storing it. Always wear eye protection and a tight fitting respirator with organic fume cartridges.

Prevail pretty wimpy
See if your store has the Crown Type … Perfect for gelcoat.

Acetone is perfect for doing spray repairs but the problem with the Prevail is you have to thin the gelcoat right on the edge of too much in order to even spray it. If you end up using it, you gotta sand / prep your repairs to 320 'cause the prevail just does not throw enough material to fill things up.

Gelcoat guys will know what I’m talking about here …

The second it takes to hit it with 320 will be better than trying to spray gelcoat that it too thinned out already in a thick-as-what-would-be-normal for gelcoat touch up film.

No need for heat if you have 65 or greater, dry conditions.

The other problem with the Prevail thing is once again the over thinned gelcoat … give it some time before you spray the PVA on it.

Recommend straining the mixed gelcoat in a paint strainer 'cause you really have to remove that little ‘filter’ thing on the bottom of the pick up tube in order to draw up the mixed gelcoat.

Mix your gelcoat with only enough acetone that the sprayer just gets it through.

Notice that pick up tube is pretty short and right in the middle of the thing ?

If its a small, EZ to reach spot. Mix your gelcoat in one cup and filter into another, use this one as your ‘cup’ instead of that glass thing that comes with the sprayer.

Saves cleanup and you do not have to mix so much up.

Clean the thing with acetone and make sure none is in there when you spray the PVA.

acetone vs. styrene
Are there times when acetone is better than styrene or t’other way 'round?

IMHO … Forget the Styrene …
Minor advantage if going for a little smoother spray for say a huge project, but smooth is relative with gelcoat and thats it I feel.I don’t like the cure times and surface hardness ( softness ) +++ The fumes just linger and linger … Horrible stuff.