After getting another coat onto the front and back with the amber, it looks like it's reacted (badly) with the Timbermate. I've read varying advice on needing to use the Behlens vinyl sealer first, it looks like the amber has not bonded with the Timbermate and left some sizeable gaps on the grain.

If anyone's using Behlen's I'd definitely either use the vinyl sealer or look at a different grain filler.

I'm going to run with it for now as I don't mind the body looking a little aged so will press on with the clear coats. If it goes pear shaped I can always sand back and go at it again!