Finishing a piece with oil and paint. How do I do this?

Hi all,
I am a complete newbie when it comes to this, I loved wood work in high school but haven't done any since. I am building a 5 year anniversary present for the wife, nothing too technically challenging at all but I have zero idea on finishing. I've built a giant ruler out of tassie oak to measure the kids on. It's just a piece of dressed timber which I've routed the edges and then used a dremel to cut grooves in at 10 cm intervals. Once finished I plan on putting on some nice brass numbers at 50, 100, 150 and 200 cm and mounting it on a wall. My questions are:

- I want to fill these grooves with black paint and then oil the entire thing but I am concerned about what paint to use in the grooves? I was thinking an oil based enamel, such as the little craft pots you get at hobby shops.
- Do I need to prep these grooves to prevent the enamel oil base leaking into the wood, and if so, will it effect the oil stain if I muff it? I am thinking of just going Scandinavian oil for the finish.

I am completely open to ideas if I am way off base with these plans!

Cheers,
Andrew

Cut a test groove on a piece of scrap, paint the groove to see if it leaches (probably won't). Let the paint dry for a couple of days, oil the scrap and see what you get.

And welcome to the forum!

I did something similar a few months ago with having a black stripe at the bottom and an oiled finish up top of a box. I painted the bottom black strip first and then oiled. I used oil based enamel paint, the solvent for this is turpentine. Then when I applied the oil finish (which is heavily loaded with turpentine) it started to eat away at the paint and my pad applicator became loaded with black paint and spread black everywhere. So next time I vowed to oil first THEN paint carefully without colouring outside the lines.

Thanks FenceFurniture and Kuffy, good info. I am going to oil the ruler, sand out my grooves, use an oil based undercoat and then use an oil based enamel over the top. Thanks again for your help, I'll let you know how I go!

Milk paint would not be affected by the oil. I use oil over milk paint on the chairs I make.

mask the grooves then fill with black tinted epoxy.
finish plane or sand the entire surface
then do your oil treatment