In order for clear coat to adhere, the surface should be sanded and prepped. If not prepped right, it will peel. Sanding and prepping will cause you to lose some of the desired effect.
I have used acetone and BOILED Linseed Oil in the past, with good results. It leaves a semi-gloss sheen. 50% Acetone,50% BOILED Linseed Oil. Do NOT use regular Linseed Oil. It will take a month to dry.
Simply take the Acetone and BOILED Linseed Oil and mix half and half. Wipe on with a rag or brush, let it sit for 20 or 25 minutes,then wipe it lightly with another rag to remove the excess product. Let it dry for 12 to 24 hours, depending on the temp.
Before and after
What if any surface prep is necessary for the linseed oil treatment? I noticed your sign has some white, possibly chalky areas that aren’t there after the linseed oil is applied.
Asking because the paint on my pump has quite a bit of white chalkiness in areas, and some rust stains I’d like to remove.