How to Paint Exterior Trim Properly

A woman standing on a ladder painting garage door trim.
What You'll Need
Paintbrushes (various shapes and sizes)
Sandpaper (various grades)
Filling knife
2 -part epoxy wood filler
Wood hardener
Exterior primer
Exterior undercoat
Exterior gloss
Brush cleaner

A few years ago we purchased an old house with timber windows that was in need of some TLC. The paint was flaking off, the wood had become weathered and gray, and on first sight it looked like the windows needed to be replaced with new ones.

However, on closer inspection, and with a bit of work, we found that the timber was actually perfectly fine in the majority of the windows, and any rotten areas could be sorted out without the need for a complete new window. Modern paint technology and wood fillers have given us the opportunity to keep period timber features from being exposed to the elements and, even if they are, they can be sorted. In this piece, I'm going to talk you through how to approach the situation, which will help ensure the damaged wood is rectified and finished to a very high standard. This will not only save money, but keep the property materials original and as designed.

Step 1 - Check the State of the Timber

Check to see if the wood is rotten or not. To do this, I used the "thumbnail test." If your thumbnail disappears into the wood, then it's rotten, but if it's firm—you should be OK.

Step 2 - Strip the Paint

Removing paint from a wood window frame.

There are two ways of doing this, using either a chemical paint stripper or a hot air gun to melt the paint off. Both approaches need a little bit of care. The fumes from both can be unpleasant, so ensure that the area is well ventilated and that you wear a suitable face mask. In both cases, once the paint has softened, scrape it away using a paint scraper. Patience is required during this process, and it's my least favorite stage of the work. Once the majority of the paint has been removed, use sandpaper to rub down using a coarse grade paper, moving to finer grades to leave a smooth finish.

Step 3 - Remove or Make Good Rotten Timber

If you have areas of rotten timber that are bigger than 1/2 inch or so, then I'd cut or scrape it out and replace with an epoxy based two-part wood filler. It mixes easily and is easy to work with. Once mixed, apply with a filling knife. Allow to go hard and then sand to a smooth finish so that it's flush with the good surrounding timber.

Smaller areas of rot can be rectified using wood hardener. This is a liquid product which should be applied using an old brush (the brush will also go hard once used). The rot will absorb the liquid, so keep applying 3 or 4 times until it won't take any more. This only takes a few minutes to go hard. If the finish is less than smooth, then apply wood filler as per above at this stage. Rub down accordingly.

Step 4 - Paint

Painting wood trim.

The choice of paint is up to you, but I'd suggest going with a primer/undercoat/gloss that is made by the same manufacturer. Some of them now class the painting products as "systems" in order to help make sure that you choose the right products. Whatever you choose though, there are generally either water-based, or solvent-based product ranges. Follow the manufacturer instructions for application advice, such as the number of coats, and drying/re-coating times. I've always painted two coats of each primer, undercoat, and gloss. Each subsequent coat protects the previous. Other decorators I've spoken to only apply one coat of gloss, but I think two coats gives a better finish and also protects against any missed areas. For a really great finish, rub lightly with some fine wet and dry sandpaper between coats.

In following this step-by-step process, the timber should last for many years. Some of the paint manufacturers' products come with guarantees as to how long the paint should last before needing to be redecorated. In general, the longer the guarantee, the better the quality of the product. It really is worth spending a little more here as it will pay off long-term.