Fiberglass doors have a realistic wood look, but staining them improperly will ruin it. If you want to apply a new stain, start by completely removing the topcoat. (Therm -Tru Doors/Therm -Tru Doors)

Q: I have a Therma-Tru fiberglass front door that a friend recently re-stained using the company's stain kit. The door was sanded (I believe lightly with fine sandpaper) but not stripped. The exterior side turned out blotchy and is totally unacceptable. I think the door needs to be stripped and re-stained, but how do I find someone to do that?

Alexandria

A: Finishing a fiberglass door, whether you use a kit like Therma-Tru’s or buy the finishing materials on your own, is a two-step process: First you add color via an oil-based gel stain, then you top that with a clear finish, typically a water-based one designed for exterior use.

It’s possible to refresh the clear topcoat once it becomes dull. Therma-Tru recommends prepping the old finish by washing it and letting it dry, figuring that a dull old finish is rough enough for the new topcoat to adhere. Other manufacturers offer different advice. Jeld-Wen, for example, says to scuff up the old finish first with superfine steel wool (often labeled as 0000).

It’s also possible to touch up small spots where the color has been scraped off. But this type of touch-up has to be literally just that: Touch a small brush dipped in a bit of stain onto the bare spot, then lift the brush away. If you try to rub on stain without completely removing the previous topcoat, you will end up with a mess — as you’ve experienced. Wherever the old clear coat remained after the door was lightly sanded, the oil-based stain just skated on the surface. “Trying to apply new stain to the topcoat — it does not adhere,” said Ladonna Bowlin, a customer-care agent for Therma-Tru Doors in Maumee, Ohio (800-843-7628; thermatru.com). “It looks blotchy.”

The only solution, at this point, is to strip the door and start over — provided you have a door made by a manufacturer that verifies you can use a stripper without ruining the fiberglass. Therma-Tru says certain strippers can be used on its doors, while Jeld-Wen warns against using any type of stripper or solvent. For other brands, check by calling the manufacturer’s customer service number.

Therma-Tru recommends only methylene chloride strippers, which are quick and effective but not kind to the environment or human health. Methylene chloride is a greenhouse gas, so it contributes to global warming. It’s also considered a potential carcinogen, and even short-term exposures to high concentrations may cause mental confusion, lightheadedness, nausea, vomiting and headache. Kyle Rhine, a marketing specialist at Therma-Tru, said the company recommends this stripper because it is effective at pulling stain out of the texture that simulates wood grain and because it is compatible with the fiberglass material that the company uses.

If you use a methylene chloride stripper, work outdoors if at all possible to guarantee enough fresh air. Where ventilation is limited, even wearing a half-face respirator with cartridges suitable for organic vapors isn’t much help; although methylene chloride is an organic vapor, it works its way through these cartridges very quickly, so you can’t depend on them to protect you. Methylene chloride can also be absorbed through the skin, so wear chemical-resistant gloves.

Adding to the complications of stripping, Therma-Tru recommends leaving the stripper on for only three to five minutes, because a longer time might also strip off the factory-applied primer. To stay within the time limit, apply stripper to only a small area at a time and wipe away the residue immediately. If you need to, use fine steel wool (000) to get residue out of corners, but be quick and very gentle so that you do not scrub away the faint texture that gives the door its realistic wood look.

If you find yourself wiping off ash-gray residue, or rose-colored material if your door has a mahogany stain, that’s a sign that you are into the primer layer, in which case you can completely strip the door, wash it down thoroughly and then start over, working your way up from a fresh coat of alkyd- or water-based primer, topped by stain and then a clear coat.

Or, at that point, you could skip the stain concept and just paint, using the same kind of water-based paint you’d use for any door. Paint is also the solution for homeowners who don’t like the look of fiberglass doors made by companies that warn against using any type of stripper, since it is impossible to re-stain these doors.

As to how to find someone to do the work, try calling house painters. Make sure the person you hire has experience with fiberglass doors or reads the care instructions from the manufacturer. Therma-Tru’s advice about finishing and refinishing starts on Page 24 of the owner’s manual, which you can download from the “homeowner resources” section at thermatru.com.