Painting spar dash

For unpainted spar dash, I wouldn’t automatically use stabiliser. Stabiliser is generally only needed if the surface is chalky, powdery or friable. If the dash is sound, clean it thoroughly, treat any algae/mould and allow it to dry fully. Then apply a thinned first coat of good-quality masonry paint as a sealing coat, followed by two full coats. Spar dash is very porous and textured, so allow for considerably more paint than you would on smooth masonry.