Before painting the frame you need to check whether there are any knots visible in the timber. The danger with knots is that with time they can start to ‘bleed’ or show through the paint (especially light-coloured paint) as a sort-of stain. One solution is to seal the knot with a patch of ‘knotting’ which is a product based on shellac, marketed by companies such as Rustins and Colron; it dries in just a few minutes or less. Alternatives are sanding sealer or pure shellac, both of which will work in a similar way. Or simply brush on some woodworking glue but be careful to minimise brushmarks.