You could get a tin of "Liming Paste", Potty........ or "Liming Wax"......
http://www.briwax.co.uk/products/limingwax.html
Or, this kit.............
Amazon.co.uk User Recommendation
I've just used emulsion or water-based undercoat in the past.
A light rub down for the doors to remove grease and shine...
Wipe on white emulsion with a sponge or piece of cloth...
Leave to dry, then rub down to remove surface paint... leaving the "lime" in the grain.
Seal with clear wax, clear varnish, or floor sealer.