New information!
(I'm calling this the "long series of c*ck ups" board)
I've attached some photos to illustrate what I'm talking about.
I have discovered there is quite a difference in how dry film photoresist behaves, depending on how old it is!
It wasn't the wax causing the problem, well it was a little bit. What I've discovered is the old photoresist I was using which is quite old and has gone a pale violet colour, works really well if it's wet. It loves water and a good hot laminator.
The new photoresist I've got is much fresher, and it's blue, much bluer than the old stuff ever was. It likes slight dampness, and a lower temperature laminator. With a completely dry board, I was able to re-position the film, but on the other side I tried breathing on the board before applying the film, and it stuck down quite well before even putting it in the laminator. So now I'm having to wind the clock back in my dry film process.
All this took 5 attempts to find out. Wasted quite a lot of photoresist, with two sides, glad it's a small board.
So the next thing, etching, again didn't go right. Thin copper left between the tracks in some places which I couldn't see until way too late. I've had it before with this copperclad so it may be a problem with the cheap crappy stock I'm using. Of course I only found this after I'd stripped the resist off and done my short/open tests. Colouring in the tracks with a Sharpie and sticking the board back in the ferric chloride cured that. Harder to clean though.
Something that went really well and I'm really pleased with, all the signal vias are conducting, all the pads with connections on both sides connect through the board. Can't individually test the ground vias but I put lots in to make up for any duff ones. So the wax has done a good job there. Last board I did wasn't tented properly on some pads so the plating got etched out.
The solder resist (the main point of the whole business with the wax, in case you forgot) went on really well, no problem with it going through the holes, except the corner ones which I took the wax out of. No more air bubbles than for a single sided (therefore, not pre-drilled) board.
However, more trouble with that, over-exposed the component side so some pads have resist on them, and the board mustn't have been clean enough with all the messing about as some cured resist peeled off with the cover sheet.
So I just need a better hole filling material and some better methods...