What's that you don't see?  Nail holes!  I went nuts to get perfectly smooth trim when we replaced all of it in our house mini-remodel.  It seems so simple, so easy to putty and paint, but those holes are really hard to hide completely (really, look at your friends' houses).

Here's the step-by-step guide on how to get deliciously smooth trim.

New trim before and after
Before & After
new trim
New trim

I filled about 800 nail holes, caulked 850ft, and painted about 600ft of trim.

The Headache

The problem with common trim is that it's made out of MDF (medium-density fiberboard), so any bit of water will make it balloon out like pressed wood.  You can't simply slap on pink putty and paint.  This painstaking method takes 3 days to complete a section of trim, but the results are soul-satisfying.

Step-by-Step Process

1. Spray or brush paint the MDF boards.  They're usually already pre-primed but a good coat now will make it easier on the baseboards since you won't need to cut perfectly to the floor.

How to fill nail holes: spray paint
Setting up boards to paint.

2. Cut the baseboard or trim piece to length

How to fill nail holes: cut trim
Trimming to length.

3. Nail gun the baseboard or trim to the wall

4. Caulk the seams. From here on out, use a headlamp all the time to see small imperfections. Amply caulk a length, then run a wet finger over it to wipe away excess. You'll waste about half the caulk. I kept a small mug of water to re-wet my hand and a damp cloth close by.

How to fill nail holes: caulk
Headlamp FTW.

5. Sand the nail holes since they mushroom out from the nail gun.  I used 220grit sandpaper on a sanding block.  

6. Vacuum dust out of nail holes and wipe down with a slightly damp cloth.  If you leave the dust from your sanding in the holes, it'll expand when you putty the hole and you're back to mushroom.

7. Put lightweight putty in nail holes and let dry overnight.  Don't use the heavier pink putty--that works better in drywall.  The lightweight putty will mushroom less. Note: this putty dries out fast so get the small container.

How to fill nail holes: putty
Apply lightweight putty.
How to fill nail holes: putty
Scrape away extra. Don't worry about getting it perfect. You need to do this a 2nd time anyways.

8. The next day, sand putty holes with that sanding block and 220 grit sandpaper.

9. Repeat putting a coat of lightweight putty on the holes again--often the putty will collapse in and won't be perfectly flat.  Let it dry (less time than overnight) and sand again.

10. Lightly wipe away dust with damp cloth.

11. Run your fingers over the trim with your eyes closed.  If you can feel anything, you'll be able to see it when you're done.  Re-putty if needed. Here's what a ready-to-paint hole typically looks like.  The edges of the mushroomed nail hole are sanded off, the putty is perfect with the wall, and your pre-painted board is pretty sanded.

How to fill nail holes: ready to paint hole

12. Put oil based primer over the puttied areas and exposed ends of MDF to seal it. Let dry.  The oil seals the exposed pressed fiberboard, else the latex paint will mushroom it again.  

13. From here, use your favorite method to get a perfectly straight wall-trim paint line.  I did it with tape and a double-cut method since painter's tape doesn't block paint perfectly, especially on textured walls.

Ex: Put painter's tape on the wall at the seam. If your wall is red and trim white, paint the edge of the tape with red to seal the tape. This will prevent the tape from leaking when you paint white on it (and every painter's tape leaks, no matter how good their marketing).

Paint the trim with 2 coats of white, being careful not to paint your newly refinished hardwood floor!  Peel off the tape while the 2nd coat is still wet.  I found it best to peel off the tape at a very sharp angle.

How to fill nail holes: paint technique

It was a ton of tedious work but the crisp lines were worth it!

If you're in the San Jose area, FMD Distributors (floor, moulding, doors) is a great place for material at contractors' prices, a fraction of the cost of Home Depot or Lowe's.

Headlamp. The Black Diamond Spot puts out 300 lumens. Climbing headlamps like this are key. Also very handy for attic work.

Amazon link

Face mask for vapors. I use one all the time for painting, esp with oil-based paints. Also handy if you're ever working in the attic with blown-in insulation.

Amazon link

Sanding block. One of those things you'll find more and more uses for.

Amazon link