Being cautious I advised about 25% vikleen in warm water to start but you can use it neat if the surface will stand it and you are very careful (wear marigolds). It is an industrial detergent so skin protection is advisable. I suggested this dilution rate just in case it got onto someones nice pollished paintwork as it is very good at removing the shine if applied neat (good prep for painting). A bit of experimentation is called for I think, I personally mix it 50/50 in a spray when cleaning vinyl and use an old nail brush to get into the grain, then wipe off with a damp cloth.