What is Cement Particle Board (CPB) sealer paint

Cement boards are highly alkaline and will burn most paints clean off them. You need something that's intended for use over masonry. There are special primers for this, but if you're putting faascia boards up above a wall already painted with Sandtex or similar, just use some of that.

Never noticed that. I've painted Aquapanels with emulsion and oil based undercoat without incident.

Water-based emulsions may or may not work depending on the resin. Old traditonal paints will generally work fine, modern resins may not. Oil-based paints are the ones that really have the problem.

I'm mainly thinking of Viroc Versapanel (the one I use). For Aquapanel, they take a halfway approach:

"Painting - no pre-treatment is required for water based paints, whilst oil based paints require an alkali primer. "

Many thanks for all your replies but just in case the link I gave isn't working (which it wasn't the last time I checked), below is the paragraph I am referring to. Infact I am using 18mm WBP ply. Can I just seal the UNDER with EVA or PVA?

"What if I have just ripped up my floorboards, and I want tiled floors? Do I have to put down floorboards again? Get 18mm ply, WBP (Water and Boil Proof) minimum, marine or flooring grade are suitable. If this will cause a step downward, you can obviously source thicker plywood. Paint at least the edges and (soon to be) underside with Cement Particle Board (CPB) sealer paint. " --- from