Black streaks (marks) in white gel coat

Dear community, as you have been very helpful in the past, maybe you can help me with this one.

I make parts for pickup campers and on my most recent ones mysterious black streaks have been turning up after pulling the piece.

Steps: Black Vinilester mould, seasoned, chemical release agent. 1 layer of white polyester gel coat @ 1.2-1.5% MEK brush application, 2h, then 2nd gel coat on edges, 8h, then 2 x CSM 125, 2h, then 2 x CSM 450, 12h, pull piece.

The finish is great apart from these annoying black marks.

Any ideas? Photos attached.

Thanks in advance!

 

Those marks look like bristles that the brush has shed.You may have lighter bristles that don't show up so visibly.Higher quality brushes would be a step in the right direction and if you are sufficiently determined,the errant bristles can be dug out and a small amount of white gel applied to the groove that results-prior to rubbing down and compounding.

 

Looks like brush marks from the gel coat being too thin.

 

There is definitely some of that,the area I have outlined in red is one example and the area I have outlined in blue looks like a stray bristle.

 

Thanks @wet feet and @ondarvr ! I agree, bristles and thin coat were also one of my thoughts, but what speaks against that is the black colouring, as the bristles are beige/brown and the resin I apply afterwards white. I am diligent cleaning the brushes before use, but will try with lighter bristles and better quality brushes. One thick gel coat layer has worked well for me in the past (two on edges we sand afterwards), would you always apply 2 @ondarvr ? Thanks!

 

@wet feet , thanks, just saw your pic with the outlines.

 

Since the resin is white, I'm assuming you added pigment. Pigment can separate and /or be filtered out by the glass as it flows through while wetting it out.

And since one layer of gel coat is commonly not sufficient for uniform coverage, I'd still go that route. And every batch of gel coat may not perform exactly the same, same with the resin and the pigment used, so there are several variables that are likely contributing to the problem.

 

@ondarvr @wet feet , thanks again for your feedback, it is really appreciated. Next week I will try with better brushes and a second coat of gel coat and report back.