If you want to re-finish trim, it's easier to remove it from the wall than to work in situ. (If you're just painting, no need to remove.) And that means carefully numbering each piece so that it goes back where it belongs. It's a lot of work so unless your trim is very high quality, you're better off replacing it.

Your painter is correct about the oak grain showing through. It's not a terrific look. If you're set on doing it though, you can try this:

1. 1. take down and number trim.

2. 2. sand face of trim.

3. 3. coat with sanding sealer which will mitigate the oak grain.

4. 4. sand again.

5. 5. possibly repeat steps 3 and 4.

6. 6. paint.

7. 7. re-install trim and retouch any nail holes.

9. My house has reddish wood trim and doors. When I moved in my first thought was "That HAS to go!". But the issue was that the walls were painted a cool grey that clashed horribly with the trim. When I repainted the walls a warm cream, all of a sudden the trim was fine. So consider whether changing the wall colour will make the trim "problem" go away. Painting walls is a lot less work than refinishing trim.

Test the results in an inconspicuous area. The trim on the back side of a closet door, pantry door, etc. You may fall in love with the look. Thousands of customers do.

There is nothing wrong with a wood grain finish if you like wood grain. Perhaps your painter isn't confident in his/her prep skills? Or doesn't know how to price it.

If it's indeed real oak trim;

1. Wash surfaces with Dirtex and allow to dry.

2. Sand with 180 grit sandpaper and vacuum clean.

3. Prime with STIX primer and allow to dry.

4. Sand lightly with 220 grit sandpaper and vacuum clean.

5. Caulk all gaps at walls and joints. Tool neatly.

6. Apply two full coats of PPG Break-Through in SW Alabaster color.

Painted oak looks just fine. The grain WILL show a bit. But hey! Then people will know you don't have crummy MDF!! :)

No to MDF board. It will warp if any moisture comes in contact with it.

Wow! I am overwhelmed with gratitude at all this incredible knowledge and advice!!! Thank you everyone!

"No to MDF board. It will warp if any moisture comes in contact with it."

Absolutely nothing worng with MDF Trim. I would use it for casing and or crown and not base as it is not as durable with vacumns and teh abuse baseboards can be subject to. And it will not warp from moisture contact. Were it get soaking wet repeatedly it may fall apart but it will not warp.

If it's getting wet, you have other problems that are more serious than what your trim is made of...