Put on at least two coats of undercoat as that board will suck up the first coat and leaves blotchy finish coat.
See how two coats of undercoat looks, if it is blotchy put on another coat. The same goes for the final coat.
You can buy size which seals the surface, two coats of that and one coat of undercoat should be enough.
You buy size in the paint department it is thinner than paint and sinks in well.
Whatever you use check the product by doing a little area first, I would suggest the area that gets hidden by the trunk roof liner.