I am having a problem with GC50 and shrinkage.
I've noticed shrinkage before in sharp right -angled corners, where the gelcoat has pulled away and out from the corner, often following one surface or the other leaving a void one side. Sometimes a bigger void one side and a smaller the other.

This time I have seen a fairly large shrinkage, shown below. I gave a thicker than normal coat of Sprayed GC50, with 15% white Polyester pigment, 3% styrene 2.5% MEKP
This piece is scrap obviously, and I've had to strip it out and start again.

I am using a gravity fed spraygun, with a 1.8mm nozzle. It's not a gel coat specific gun but does the job for me as the part is not huge. I would have thought that the gel coat gun would have provided a thicker layer anyway?

The mould is very well seasoned now, and I use TR regular wax only.

Any thoughts? This is probably the one problem I have with GC50, in all other aspects it suits me perfectly. I'd like to get on top of this, otherwise it's back yet agin to finding a epoxy compatible poletser gel coat!

Is this hit and miss so works sometime and not others or is it failing every time.

Have you tried the GC50 with no pigment in as I wonder if this is causing the problem.

This is the first time I've had an issue with this mould. I would have expected it to occur(if it was going to) on a the tighter corners, but it hasn't.

When it has done this before, it's been when mouldmaking, and in the virtually 90 degree corners between the plug and parting surface, or Dam. Then I discounted it as a uneven thickness of material where it had "filleted" in the sharper internal corner.

This time though it's across a fairly shallow deviation?

I haven't tried it with no pigment, it would defeat the point  for me to not use pigment. the pigment is Easycomposites polyester white.

How will catalyst percentage affect any shrink? I tend to use a fairly accurate 2.5% as opposed to the 2% on the tin. Would that really do it?
I just checked the shelf life. I purchased the 5kg tin just a fraction over one year ago. Fair enough it's fairly beyond the recommended shelf life, but I did have this issue with the mould making when the Tin was pretty new?

I have also noted that the overspray areas, where the layer is relatively very thin, never quite cure?

If the coat is too thick or the MEKP is too high for the temperature, then greater shrinkage than you would expect is a possibility.  It can be common in corners where there is a higher likelihood of a thicker coating being applied.  The hotter it gets during cure, the more likely it will shrink significantly.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Temperature was 23/25c, mostly 25c as I have a low-bake oven/warming cabinet that's thermostat controlled and pretty accurate.

Humidity I don't know, but maybe 60% at a guess?

I have always tended to use 2.5% MEKP

What's the minimum percentage of MEKP that would still kick to a cure? Has anyone any experience of using lesser amounts?

I have redone the moulds this morning using a little more styrene, 5% and the 2% MEKP so will see what the result is.
I've also put some tape on the back of the mould where it delaminated, and sprayed over that edge to try and create a "Tie-down" effect.

Another question on this subject of GC50.......
What's the scenario of multiple coats of GC50 to create depth? For example, I spray a coat on, then leave it for 3/4 hours and then spray another layer on top?

You can't use multiple coats of GC50 as it has been specially formulated for epoxy to bond to it, so when you apply a second coat, it does not bond well to the first meaning you are likely to get delamination.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Aaargh! Getting into a corner again!

It's a real shame as GC50 seems very useful on the whole.

Pro's
It's sprayable
very polishable to a high gloss
Takes pigment well
UV resistant
Tough finish
Cheap

Con's
Shrinks where thickness not even?
Seems to not cure when in very thin layers
Pain to clean up the gun

I have tried many versions of epoxy gel coats. They all seem to get problematical when you want to pigment them, and seem to be on the soft scratchable side when cured?