more gelcoat spraying-specific to gun/setup

Hello,

Don't mean to cross post, so figured i would post a more general query.

I am in that weird zone of rarely gelcoating, thus not wanting/able to have top notch equipment. I have a little bit of experience, having sprayed my top hull a few years ago. I would give myself a "fair" in outcome. I was a little loose in the way I did it, and had the respected results.

This time I am set up better. I have the boat in a garage where I can control temp and humidity. The challenge will be that I am spraying the bottom of the hull. The boat is on blocks and stands. I can't flip it.

I bought a cheap, but more appropriate gun. It's a chinese gun, similar to many of these type. It seems to be decently made. It has a 2.5mm tip. I'm running it off of a comp that delivers 6.4 CFM at 40. The gun needs 35-50 PSI and uses 2.5-3.5 CFM. I have a couple of filter/driers, including one right at the gun.

I initially posted about a patch i was gonna do. I needed to do the patch, but also wanted to use the area to dial in the gun. I tried the first pass at around 40PSI. I shot 6 oz, thinned with 10% MEK. I added 2% wax and 2% catalyst. Temp was around 68-70 and humidity was 55%. I had the spray control valve about halfway open and the fluid control valve mostly open.

The material went on reasonably well. The pattern looked good. I did notice a decent amount of orange peel. I got a message from the Fiberlay guys(after I shot it) that I should only thin to 5% if possible.

I am wondering if there are any obvious changes you would make in terms of conditions or mixing to try to get a smoother layup?

Thanks

Ian

 

I don't know if the pics will be helpful, but here is a closeup and another for scope

 

The cheap guns work fine for gel coat, a good one with the a 2.5mm tip on Amazon is about $35.

Gel coat isn't formulated to work like paint, so getting it to flow and level well isn't something it's going do without some help.

Duratec clear additive will help you get closer to a good finish, the more you add the better it will level and flow. You risk losing some of the gel coat's ability to hide underlying colors if you add too much though.

This amount will change with every color and brand of gel coat, so test it first.

It still won't be perfect, but much closer to what you want, so far less sanding is required.

Gun adjustment is something you need to play with, there are no exact right or wrong settings, it's just what works for that specific spray job.

 

I went ahead and sprayed the boat without Duratec. It's an imperfect situation, but overall pretty happy with how the gelcoat sprayed. I have the boat up on stands(couldn't flip it) so am having to spray at some weird angles. I'm not gonna do much sanding on the bottom, focusing more on where it is visible, like coming up the bow line. I had the best luck with the fluid control open, the gun around 45 psi(max is 50). It shoots pretty well to my limited experience, and the biggest challenge was trying to shoot upwards.

I have a couple of funky spots where the cup of the gun grazed the wet coat. I tried sanding it out in one spot and got back to the fiberglass. One spot looks like a long shallow gouge. Should i just use a gel-paste in these spots? Seems weird to spray for such a small area.

Speaking of sanding, I have a DA(on a GFCI) with a soft backing plate and a bunch of wet/dry sandpaper. I experimented along the bow line with some 600G. Was trying to see how long it would take to get any imperfections out. I found the 400G seems like a better starting place to macro, then switching to the 600 when i can barely see the shadows. Still a lot of work

 

Use PPS cup system, it will spray at any angle.

 

You might want to look into using a viscosity cup.
It is a simple gravity device that permits the timed flow of a known volume of liquid passing through an orifice.



Do not breath the fouled air.

 

Have tried the spray thing several times. Latest were some deck panels that I could hang vertically which is best. Used Duartec (followed directions) and a 3.0 tip as I was starting from bare glass. Did all the proper prep, air temp 70 humidity around 35%. Did it in three 400ml mixes with 2% MEPK (gun holds 500 but gets a little sloppy) after all was done and left to cure about six hours looked good but several tacky spots. Waited till overnight still tacky. Applied mold release wax to the tacky parts and finally cured.

This was not my first time spraying or using Duratec. But the outcome, at least in my mind, is ify. I’m not knocking Duratec but I live in the northeast and maybe it is due to not being able to acquire “fresh” product without paying exorbitant shipping fees.

I have come to the conclusion that thinning gel coat with anything you no longer have true gel but a diluted resin. I have since sanded and applied straight gel and sanded to finish. It reallly didn’t take very long and I can buy lots of sand paper for what I spend in additives.

Just my .02c

 

Depending on the exact gel coat, temperature and humidity, overnight may not be long enough for a full tack free cure.

Low emission gel coats don't cure as thoroughly or as easily as the older versions, this results in longer time frames for a completely tack free surface.

Old gel coat, or gel coat that has been pigmented for a custom match may not cure as well either.

The ability for gel coat to fully cure diminishes over time, this makes it more difficult to achieve a tack free finish. Also, pigments are suspended in a resin base that is formulated to not cure, that's why the shelf life is extremely long for pigments. When small companies or DIYrs add pigments it reduces the ability of the gel coat to cure, small amounts of pigments are fine, adding too much can severally inhibit the cure though.

Large gel coat companies adjust the gel time back into spec after the pigments have been added. Small repackegers don't have the ability to do this.

Duratec is a resin based product, so it doesn't compromise the physical properties of the gel coat. But since it reduces the viscosity it can take a little more time to build it up to the correct thickness.

 

I would also recommend using a wet film thickness gauge. $10
As I sprayed I count the seconds per foot as I move across the surface to achieve consistency in the desired thickness of product.
With a consistent viscosity/in seconds and thickness/in microns I have had a much better over all finished product.

 

Let me be clear about my post. Yes environmental variables do affect outcome I completely understand that. However when I roll the same product using 1-1/2% MEPK (manufactures recommendation) vs, adding Duratec and 2% MEPK (Hawkeye recommendation) the rolled gets to a re-coatable tacky within a couple hours under very similar conditions, Gelcoat is Ashland standard white non wax.

Again the sprayed result was still wet only in certain areas and thickness was equal to other areas cured. If it was the same all around I would have been satisfied with the results I am very meticulous about my spray patterns.

Although I admit I have more experience with paint. In comparison the panels I sprayed took approximately 6 hours to set up including over spray protection, mix, spray , clean gun and repeat and dispose of the dirty acetone.

Conversely, I rolled three coats of non wax and one coat of waxed. Sand from 120 grit to 2000 with nearly out of the mold looking results didn’t seem to take much more time.

I’ll say it again not knocking Duratec I’ve just decided it’s not for me as there is something going on I can’t figure out. Or, take the time and expense to do extensive trial and error testing which may or may not resolve the issue.

 

This isn't a shot at you, this is a common topic when refinishing with gel coat. So please don't take offense at it. It is something I've posted before.

Uneven cure can be caused by a few things, but the chemistry isn't smart enough, nor does it have the ability to cure in one area and not in another without some outside assistance. This is totally unrelated to adding Duratec, it's something that gel coat and resin tech service people get calls about fairly often.

Frequent causes of uneven cure.

Not mixing the gel coat or resin prior to using it.

Using old gel coat and/or catalyst.

Not mixing the catalyst in thoroughly. Always mix it in a separate container before pouring it into the gun. And mix for at least one minute with a good stir stick. Or better yet, a drill motor with a paint mixer attachment. This is vey important and a common problem.

Not using the correct amount of catalyst. This compounds the poor mixing issue.

Not having air moving across the surface. Styrene fumes can collect in low spots and inhibit the cure.

Uneven thickness on the surface, thicker areas will cure quicker, some thin areas may not cure well at all.

Temperature differences across the surface.

Sun, or other heat sources affecting the surface. In direct sunlight wax may not come to the surface prior to the gel coat starting to cure, so the surface will remain tacky. This can be uneven across the surface depending on the heat source.

Some types or colors of gel coat cure poorly with any additive, every gel coat company has hundreds (actually thousands) of different formulas, each one can cure differently.

Some colors don't cure well. There can be a huge difference in how each pigment impacts the cure, and the bolder the color, the greater affect it can have.

Since 99.99999% of all gel coat is used in-mold, that's where it is formulated to work. The fact that it can be post applied like a paint (most of the time) is something that isn’t targeted in the R&D work.

It isn't uncommon for some gel coats to cure poorly with any of the surface cure additives, so frequently two additives can be combined to aid the surface cure. This can be done by the person doing the work, or by the manufacturer. Duratec offers several products that have both wax and the surface cure resin in combination to achieve a quicker and more thorough surface cure. With Duratec alone you get a very high gloss finish that will level and flow more like paint, some people don't touch it after that, no sanding or polishing, that's the finish they want. Others will sand and polish this finish to make it even smoother. If you plan on sanding, then using wax in combination with the Duratec additive gives a you very smooth surface, but no gloss. And since you planned on sanding and buffing anyhow, the initial non-gloss finish is irrelevant.

Adding Duratec can reduce the amount of sanding required immensely, sometimes you can start with 800 grit and then buff it. With just wax you typically need to start with a much coarser grit to get rid of the orange peel, so the time and cost go up significantly if you want a smooth and glossy finish.

Most gel coat finishes don't improve much past 1000 grit sandpaper, and with some you don't need to go past 800 to remove all the a sanding scratches. This depends a great deal on the exact gel coat and the exact sandpaper. Some gel coats are harder than others, so it's not as easy to buff out sanding marks, softer gel coats are easier to buff to a high gloss.

Sand paper comes in many styles and qualities, cheaper sand paper may not have the same uniformity of grit size, or may have uneven layout on the backing, so it will leave uneven depth scratches.

The hardness of the grit comes into play too. Some grit wears down quickly and leaves finer scratches as you sand, others are harder and continue to cut at about the same rate. Cheap sand paper can have a mix of grit that may have a different hardness, so the scratch depth can vary, and change as the grit wears down. So cheaper sand papers may require continued sanding with finer grits to remover all the scratches from the grit irregularities.

Buffing compounds and techniques make a huge difference too. So you need to find out if you like sanding less and buffing more, or sanding more and buffing less, both methods can work.

There are fast cutting buffing compounds designed to remove 600 grit scratches on most gel coats. These compounds don't leave a high gloss, it will be glossy, but it can be made better with a slightly less aggressive compound being usdd after it. Then can switch to a foam pad and bring the gloss to a much higher level.

If you sand more with a finer grit you probably won't need to buff with the fast cut compound, you can start something less aggressive, and depending on the desired result, you may only need to use this one buffing compound.

Color can make a big difference in the amount of work required, white won't show many defects in the surface, but black will show every defect, they will stand out immediately. And the higher the gloss, the more defects you will see.

 

No offense taken. It’s my opinion you are one of, if not, the foremost experts on gels and resins on this forum. I follow all your posts with great interest. Not to beat a dead horse or derail the thread I would like to address each of your comments. My only purpose is to perhaps enlighten myself as well as others who may be experiencing the same issues. Or expose my own faults.

-Won’t be buffing.

Thank you for you reply