Composite baseboard should be relatively water-safe. If you had serious water in there, it also likely got into the drywall, insulation and everything else, so you should have concerns about mold that need to be addressed before doing anything else. I guess that doesn't answer your question, but your issue raised a red flag for me.
Lili, MDF is easily damaged by water. Anytime you install this type of base in a bathroom, or where it will be subject to excessive moisture, it will need to be properly sealed and painted prior to subjecting it to any moisture. Typically this base/trim comes pre-primed, but that's just for transit, and still needs to be coated with a good primer, and a good paint. Sealing all the edges, where it touches the floor, and where it meets the wall are very important too. I would at least
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use a paint-able latex plus silicone caulking (I like Alex-Plus) for these high moisture areas. I'm sorry to say, but once the base has swollen, you will need to replace it. The structural integrity of the base has been deteriorated, and will not last. Further damage to the wall can and will occur if the water is allowed to penetrate continually. Hope this helps. Good Luck!
To add to Imagery's comment...the pre-primed nature of this trim is only on the "face" side...the backs are not primed. Some folk paint and seal both sides before installing this helps a great deal...but the big question is ...HOW did this trim get so wet in the first place?
It is difficult to back up the deterioration process with particle board type products. But, if you wish to try and work with what you have, you need to prime with an oil-base primer (like Kilz) before you apply the latex wood filler, so the water in the filler doesn't make it swell again.
It is not in a bathroom but along a carpeted stairway. I should mention that we have an occasionally naughty little male dog (G), but he is not the whole source. I have found slight swellings at the bottoms of baseboard he doesn't get to, I suppose from 10 yrs of wet washing.
Thank you all for your suggestions, I will sift through them and make a plan soon.
I know it's a pain...but remove it, install wood baseboards and paint with exterior paint. You can use waterproof caulk to seal any open gaps between the baseboard and flooring. I hope this helps. Good luck
I was thinking of a different product with my earlier answer, not MDF, but anyway, my concern about the source of the moisture still holds, especially if it is not in the bathroom and is not apparent.
IDK what MDF is, but I get the impression that composite baseboard is a really bad idea unless prepped as some have described. If I had known, yadda.... I will definitely use the seals advised above in fixing them. (replace is not an option).
Part of the setup is that I attempted to refinish the handrails above the area without adequate protection for the base, some time back.
As to the water KMS is right, where is it coming from?
Is this trim inside near a tub or shower?
Best, Charles
Thank you all for your suggestions, I will sift through them and make a plan soon.
Part of the setup is that I attempted to refinish the handrails above the area without adequate protection for the base, some time back.
O well,
CP