Hi,
I tried repairing some damage to rudders and daggerboards on my Tiger and the short strand fiberglass filler came out quite soft. This time I coloured it with what I assume to be the proper stuff.
The polyester is probably 2 years old and the hardener and colorant cream are probably less than a year.

I mixed the colorant so the result was reasonably close to white. Is this the culprit? Or maybe was a too chep with the hardener?

I wouldn't recommend putting colorant in the resin - it only stands to weaken it. Make the repair and then paint it afterwards. Cheap and old are not two characteristics you want to have with a resin either!

Thanks Jake
I'd say the colourant is the culprit,

I'd recommend using west system epoxy materials to make repairs and get to a smooth surface - then you can either paint or go back with gel coat (make sure you understand if hardens in air - and scrub the repaired surface with soapy water first to remove the amine blush from the epoxy).

Use colloidal silica with high density adhesive filler for big dings or ground off bottoms, then colloidal silica and fairing filler or low density filler for small dings and fairing out.

Small chips in the trailing edges of boards can be easily repaired by placing a piece of masking tape on one side, blobbing on some gel coat and then putting a piece of tape on the other side to flatten it out a bit and prevent runs. A little work with sand paper the next day and you have a nice finished repair.

Chris.

mix the epoxy and resin first, then add your additives, thickeners, colorants, etc.

I've added color lots of times, its been fine. Follow the percentages and dont go over whats recommended.