Advice on removing blotches/discolouration from water based filler

Hi guys,

I applied a water based timber filler on my cedar ceiling sometime ago (before christmas) and have only got back to the job at hand (I know I was meant to get the filler off once dried but other things got in the way). I have now sanded down the filler but have noticed a blotch/discolouration appears where the excess filler was.

I have sanded it back quite a bit to remove what I thought was filler on the surface but it looks like it's quite deep in the wood. I've attached a picture of what it looks like with some turps to make it more clearly visible.

Does anyone have any advice on what I can do to remove the blotches/discoloration that has happened? Genuinely appreciate any advice, tips or tricks to remove these blotches!

(I now know that you're meant to coat the wood before filling, but I have clearly learned the hard way here)

Welcome to the forum.

It's probably not as deep as you think, but it has obviously penetrated enough to be a PITA.

How did you sand? A power sander? What grit? Are you planning to apply a finish? If so, what finish?

One thing you can try, which probably won't totally remove the blotches (unless you do sand "quite a bit" more) but should at least make them less obvious, is to wrap some 180 or 240 grit sandpaper around a flat off-cut and hand-sand along the grain.

Do not use a rotary motion, or sand across the grain, just back'n'forth with the grain. This process will make the grit scratches parallel to the grain, tending to remove softer sections first... which is typically the filler lying between the tubules that make up the grain pattern. Cedar is a very soft timber though, perhaps softer than your filler, so in this case it's a case of suck it and see. Unless you have some off-cuts that you can try experimenting on first?

Either way, it should only be the work of a minute or less to see any improvement... if there's going to be any at all using this method.

If you see an improvement but feel the need to switch to a finer grit, you can switch up to 320 and then maybe 400 grit but you shouldn't need to move beyond these. It's likely that when you start to go up grits the blotches will start to become more obvious again, so it's best to stop when you feel you've reached an acceptable compromise.

(Always keeping in mind, of course, that with enough sanding these blotches will be removed. But you'd need to sand a fairly long section, a foot or so, to gently blend the sanded area in with the original so that you don't have obvious sanding "dimples" from only working one spot for too long. Which is a very, very easy mistake to make with soft timbers such as cedar.)

Thanks for the welcome old1955

Thanks Andy for your advice. In response to your questions:

I hand sanded with the grain with 80 grit to knock the putty back and am planning on going to 120, 180 then 240. (I hope I haven't made a dimple like you mentioned, I know it's a pretty coarse grit!)

On finish, I'm currently planning on a clear polyurethane coat (leaning towards oil based, but also open to water based). But I've been umming and ahhhing on this, and have been considering Danish oil too. (At this stage if something minimses the look of the blotches, I'd seriously consider it)

I do have offcuts to experiment with but the hard part has been replicating the blotches I have (I think it has been a function of time more than anything)

Thanks again, looks like I have a lot of sanding in my future

Andre