use some sort of polyurethane this comes in an acrylic(water clean up) and oil based (turps cleanup) if you are going to use these oput side i recommend using the oil based as it is more hard wearing, i also recommend that either product you use air it out for a couple of weeks before putting it in an enclosure as it is a bit on the nose and very fumy, being a painter and decorator i put up with this quite often...there is also other products on the market like sanding sealers, and other types of good sealers as well you might just have to take a trip to your local paint shop to see what is available you shoudnt need alot of sealer as 1ltr should do 14 sqr metres as long as what your painting it on isnt too course. try to stay away from dulux as your mainly paying for the name , taubmans, solver ect is just as good if not better.