Defective gel coat is yellowing, repeatedly

[Bullshipper]

I bought some supposedly ISO poly artic white gel coat to produce a new boat als laid up using poly Iso resin in an open mold about 1.5 years ago . The gel was applied on the hull that was cleaned and waxed, prior to being sponque coated with PVA and the gel coat was rolled and brushed with no thinning.

This gel coat started to yellow forst on the radius curves on the cap piece about 6 months ago and after trying dewaxing the area with strong soap and water followed by thinner wipe down I was able to remove fairly easily it using 5% household chlorine straight from the bottle after unsuccessfully trying several cleaners like Barkeepers Helper, Vinegar, etc.
I believed it was coming from the tree pollen in the area or from the wax in the mold and also tried to buff with a light compound and a medium grit compound in a test area with no lick.

It me bacame back 2 months after the cleaning, and is now also showing on the lower hull panels. Theses new stains are much more resistant to cleaning unless they are wet sanded, so I am suspecting that the gel was thinned with acetone or too much styrene by the middleman and that it is bleeding out from the inner coats (2 and 3) to the top coat (1).

This is the only problem hull I have out of 6 I have produced and the bad gel coat came from the same lot.

I am prepared to re-gelcoat the hull I want to sell as new but am unsure if there is a product that I can use on the top coat to seal it prior to painting or gel coating after it is sanded by 220 in the case of paint or 320 for gel coat.

I feel it is preferable for my customers to receive a thicker gel coated hull instead of a painted hull even though its more work for us.

However, my original thought was to sand and apply an epoxy barrier coat on the bottom and top sides as I suspect this is better sealer but am not sure if I have to paint over that barrier sealer that would normally just go on the bottom or if I can successfully gel coat over that.

All my experience is with poly gel coats and my spray equipment using a 3m Accuspray HGP is sized more for repairs than for covering this 60 sq meters of surface., a 28' catamaran

So I am looking for some sage advice, as the middleman and retailer that sold me this junk are hiding out with no suggestions.

Thank you

 

[ondarvr]

There is nothing leaching out of the current gel coat, the discoloration is just the yellowing of the styrene in it.

You said it was from the same batch as the other hulls, was this in a drum, and was it mixed well before each hull was made?

Gel coat separates quickly, the heavy pigments go to the bottom and lighter styrene and resin float to the surface. This may cause different results if it's not thoroughly mixed prior to use.

If you plan to re-gel coat the hull, use 80 grit, not 320, and you'll need different gun to spray it. But painting is probably fine.

 

[Bullshipper]

Thank you.

3 pails were used the same day, and those pails had the same lot number as a clarification.

Your mixing comment could very well be the problem on my end.

As to your correction on the grit, are you implying that I should go coarser to remove most of the old gel coat down to the skin coat to start over? Its fairly soft and +80 grit will thin it fast even when using a guide coat to not over sand, especially on the redius turns,
I was hoping that I could shoot just thick enough over the already smooth surfaces to get enough thickness to sand with 320, then 600 and 1200 wet before compounding and waxing to seal.

My facility only has solar power, so I am limited to about 8 cfm for my compressor. My work around solution to that has been to store enough air in 3 tanks to shoot a 28 oz linered cup using 12-15 cfm with a 2.0 tip with air pressure forcing the gelcoat into the nozzle at any position, then letting the compressor catch up on refilling the tanks while we mix and add another cup to continue shooting.

So if I shoot around 6 mils wet, one cup will cover around 55 square feet, and 3 cups with do a side, then repeat using 3 cups more to get another wet coat to end up with around 8 mil thick when dry prior to dry sanding. I know its slower and more uneven but that's my situation.

Local painters here only use paint with small tip sizes, as no one in my area has a plural gel coat, resin, or chop gun so its not something I can sub out unless I switch to paint.

I looked into it but spending around $20K on a 30 cfm compressor with a gas engine and a metered pumps to shoot gel and resin is beyond what I want to invest as a retired small time guy, but again, I am sure open to suggestions if you have a solution for my parameters.

Thank you again

 

[ondarvr]

The need for 80 grit is because gel coat needs an aggressively scratched surface for an excellent bond, 320 doesn't do it. The 80 grit scratch won't create a problem in how smooth the gel coat lays down over it.

You don't need to sand through the gel coat, just sand enough so all glossy areas are removed.

You will need to apply closer to 18 to 20 mils for the gel coat to cure properly, plus you'll be sanding off most of what you put on to remove the orange peel.

 

[Bullshipper]

Again, thank you very for your expert advise.

I now understand why your corrected me on the futility small gun for this quantity of material and need for coarser sand paper.

So if I want to gel coat and really do not have the right equipment to spray that thick over a large area, I believe the only option I am left with rolling and tipping it thicker in two coats on the same day with a brush in corners, then running a smooth foam roller or Wooster brush lightly over the surface to smooth it as much as possible prior to sanding. to smooth it

I will assume you are also implying that after a 18 months,
I imagine that I would just follow the new gel coat prep instructions per their TDS . hoping that the styrene has probably done its cure out on the old gel coat and that the new gel coat will not let it bleed though it if well mixed
Or should the surface also be treated with a particular wipe down product other than acetone prior to the new gel application?

In any case, I will be buying my gelcoat factory direct from Reichold in Mexico City from now on to eliminate middlemen here with no scruples.

or subbing it out out with the existing gel coat, using the bottom primer to paint after sanding and prepping per that Imron or Axelseal TDS procedure.

 

[ondarvr]

I will assume you are also implying that after a 18 months,
I imagine that I would just follow the new gel coat prep instructions per their TDS . hoping that the styrene has probably done its cure out on the old gel coat and that the new gel coat will not let it bleed though it if well mixed
Or should the surface also be treated with a particular wipe down product other than acetone prior to the new gel application?

.

There is no styrene to escape, it doesn't move around once the gel coat has cured. Yes there may be some unreacted styrene vapors escaping for a very short time after it's cured, but nothing more. The "bleed through" concern is not relevant.

If spraying isn't possible, try rolling on two coats, letting each one become hard enough to not come off on your finger.

Then apply a third application with a squeegee. This fills the low spots without adding anything to the high spots, so it reduces the amount sanding tremendously. You'll need to do a couple test spots to perfect your technique before doing the entire project.

Also make sure you catalyze at the correct ratio, over or under catalyzation can result in a poor cure and yellowing.

 

[Bullshipper]

Excellent tip and advise

Thanks very much

 

[wet feet]

I would think spraying with 8cfm is a bit optimistic.Could you hire a diesel compressor for a day or two?Whichever way you proceed the finishing won't be a trivial job,but the improvement could be a big one.