Thickness Problem Spraying Gelcoat

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#1

Hey guys,
Today I decided to be bold and take a stab at spraying gelcoat over a mold. I used a LVLP gun with 1.8mm tip (sprayed @ 45 psi). Fiberglass store sold me this and told me it would be fine for gelcoat.

After spraying the gelcoat, I like the uniformity it produces over the brush. My problem is that I can’t achieve the 20 mils thickness. It seems like I couldn’t even measure it with my thickness gauge.

Also I noticed for a small mold(~18"x18"), if I brushed it, I could get 2 coats of gelcoat in (way thicker than spraying but not 20 mils)while only using 6 oz of gelcoat. It took me a total of 12 oz of gelcoat just to get what seems like a surface coat over the mold.

Is this due to the 1.8mm tip? Overspray that I’m losing material? Is it dues to the gun being an LVLP?

Thanks in advance

#2

Ahh the grand wastage… Spraying any material results in partial loss into the atmosphere. I believe LVLP guns can have as much as 25-45% wastage, which is consistent with what you are experiencing. HVLP guns are much better and can achieve as little as 10% waste.

I am surprised that you’re able to spray gelcoat with such a small tip.

#3

So it is the overspray. I noticed there was quite a bit. Spraying with a 1.8mm tip was hard since this my first time ever using a spray gun. Could barely tell if anything was being sprayed at all. Looked like just air was coming out.

So would you all suggest a gun with a 2.0 mm tip in hvlp?

#4

Gelcoat is thick, too thick in my opinion to spray with conventional guns that gravity feed or siphon feed (even if the salesman says it works). You need a cup gun (aka dump gun) or a pressure pot. My cup gun has a 6mm tip that I use for small parts. On larger molds, like a boat hull, if I had to use a cup gun, I’d probably opt for something even larger. My pressure pot has a fluid nozzle that is 2.8mm. A pressure pot pushes the gel out with about 30 lbs of pressure so you can imagine how much gel comes out in comparision to a regular spray gun. I can spray a gallon of gel in less than 7 minutes while carefully applying it. If I was just squirting it out it would probably be less than 5 minutes.

The cost of better film thickness control and less porosity is lower transfer efficiency (the amount of gel that actually stays on the part). Cup guns and pressure pots have a transfer rate of around 50-60%.

Now, if you really want to spray some gelcoat, you get a spray rig from MVP, Glasscraft, etc that can put out about 7lbs per minute (about 3/4 gallon) with high transfer rate (up to about 80-90% with modern equipment). The main drawbacks to these guns is they are better than $6000 for a new unit and they are not really made for spraying a few ounces at a time. They are, after all, production equipment.

#5

That’s more like what I was expecting ^

With the extra material wastage, time spent cleaning equipment, and the health risks of spraying, this is why I brush as much stuff as I can. I even prefer to apply my 2k high build and clear coat with a brush (though admittedly I have only done this on one project so far) despite the extra time it takes to flat and polish it afterwards.