When using exterior linseed oil paint on a wooden surface and to gain the benefits of using linseed paint, it is important to remove modern plastic paint first.
A lot of people find it hard to establish whether their project has existing modern plastic paint or linseed paint. Fortunately, there is an easy way to test this: wipe the surface with some denatured alcohol (methylated spirits) using a cotton bud.
If the paint comes off on the bud, it is modern paint, and we always advise removing this before painting with linseed paint.
If it does not rub off, you are looking at a traditional paint which, after a quick sand down, can be overpainted using linseed paint.