By the looks of your first photo, you can still see the plywood grain patterns through your primer.
I'd be applying another couple thicker coats without sanding in between to try and build the layers to "fill" or minimise the grain effect - so that it isnt visible underneath your decals. maybe even skim these areas with some finishing putty / bog (2 part).

As for the wood sealer, its sprayed on, just go speak to an automotive paint shop (a good one) and describe to them what you're trying to achieve. Tell them you're painting a kitchen and want a sealer for underneath the primer you'll be using.