Dull patches in padded shellac finish

[silentC]

I've had a couple of problems with a padded shellac finish where there is an area that is flat rather than glossy and the colour is lighter. If you look at the area with light shining across, you can see small ridges of finish surrounding the area. It looks as if the shellac has retracted from the area after application. The surface looks uniform when the coat is first wiped on. It will affect one surface out of several, so for example I have just finished 12 3" x 18" boards and one of them is affected but none of the others.

The process I've used is as follows:

1. Sand to 400 grit
2. Apply a coat of sanding sealer (just a very weak cut of shellac about the colour of tea)
3. Wait 24 hours then sand with 600 then 800 grit wet and dry
4. Wipe on several coats of shellac with short drying time in between

It's difficult to tell when the dry area forms because it's not really noticeable until a few coats have been applied as the surrounding area becomes darker.

I'm able to fix it by sanding back with wet and dry and soapy water, allowing to dry, then applying the padded coats again.

It's obviously some sort of surface contamination, but I don't know what it might be. I'm using 95% Ethanol. The affected area tends to be about the size of the pad but I'm not sure if that's a red herring.

Anyone seen it before?

[conwood]

All it takes is a momentary lapse of fluid movement. Other than that it seems rather odd. One thing is certain, the more coats the more it will magnify.

Keep us posted.

cheers
conwood

[silentC]

I just thought of something. I didn't leave it long enough between coats at one point and there was a slight drag as I applied the pad. What I do is start about 6" in from one end, wipe towards that end, then come in about 4" from the same end and wipe the full length in the opposite direction. The dull area roughly corresponds to where I might have landed for the first swipe towards the short end.

I suppose I expected that the wet coat would settle in and lay out flat, which it seemed to. Do you think that might have been the cause? Seems odd though because it doesn't seem to explain why I have that one area that just wont take on the finish in subsequent coats.

[conwood]

It's a fine art starting. As for that one spot..well that is weird. Your surface preparation seems very thorough.

Unless, the timber has somehow been contaminated before you got it. Messy glue ups can haunt like that.

All seems rather odd ????

conwood

[ubeaut]

Most likely your application technique rather than contamination. Possibly too much shellac even too strong a mix.

Are you using actual "Padding Shellac" or just shellac applied with a pad. There is a difference. If it's Padding Shellac where did you get it from. Are you sure it's fresh? What are you using to thin it with? Padding Shellac as a rule isn't thinned with metho.

[silentC]

I'm mixing it up from flakes. I ran out of UBeaut one weekend so I bought a jar of Master Touch. It doesn't have a date on it but would only be a couple of months since I bought it.

The cut would be about 4lb. I dissolved the flakes in a jar by adding metho until it just covered the flakes. Then at a later date I increased the volume by about 3 times with metho. I know it's not exactly scientific.

Wasn't aware that you could thin it with anything else. I'm just putting the above mix in a flip top bottle that once had UBeaut hard shellac in it. I've made a sort of rubber which has cotton wadding at the centre, an old sock around that and then a bit of cotton bed sheet. I open the top and charge the pad. If you squeeze the pad you see a bit of polish well up behind the sheet around the edges of the pad inside but it doesn't ooze out. I usually charge it up once before I start and that seems to be enough to do several coats, so maybe I am putting too much in at first.

What I don't get is why it is only this one board out of 12 that has the problem. It's wasn't the one I started on or finished. Its in the middle somewhere. I accept that it's probably something I'm doing rather than contamination but I can't work out why I don't see it on the rest of the work.

[ubeaut]

Probably cut the polish up to 50% more. If it was made from flakes then it's fresh from the time you made it.

After sanding back does the spot come up in the same place again? If so it probably is some sort of contamination but in theory shellac sticks to just about anything with the exception of a massive silicon contamination.

Is the timber new or recycled?

[silentC]

OK I'll try it a bit thinner.

After sanding back and then re applying the finish, the patch is gone - it has only happened three times now, I haven't fixed the third because I thought I should try and find out what is causing it.

The timber is new and has been through the thicknesser to bring it down to 3/4" from 38mm.

Do you think it could be because of that dragging I mentioned in post 3? Seems unlikely to me but that board was the one it happened on I think.

Wish I could take a photo of it but you can only see it up close with light raking across.

I'll see what happens with the next batch any way. Will be doing some more this weekend.

[ubeaut]

It basically sounds like what is known as burning the finish. If the rubber has a bit too much polish, a bit too strong and is allowed to stop, or work too slowly over a section of the finish even for a split second, it will drag and burn into the below surface more strongly than it should, thus creating a bit of a dull patch.

May happens if the preceding coats are a little on the green side and is often where the rubber touches down on the surface if it isn't smoothly glided on to the work. Especially if the rubber is too wet.

[silentC]

OK, that sounds like the problem. I should wait a bit longer between coats and I've probably got too much polish in the pad. What I find is that the first three or four flash off really quickly and by the time I've gotten to the end, the start is dry enough (or seems it). Then I start leaving it for 20 minutes or so between coats. But the problem is occurring early on, so I should force myself to go and do something else instead of standing there with the pad in my hand counting to 10.

Thanks for the help.

[RufflyRustic]

Sorry Silent, but I gotta say it...

...stand there with the pad in it's airtight container in your hand counting to 10.....

[silentC]

Point taken!

[munruben]

OK, that sounds like the problem. I should wait a bit longer between coats and I've probably got too much polish in the pad. What I find is that the first three or four flash off really quickly and by the time I've gotten to the end, the start is dry enough (or seems it). Then I start leaving it for 20 minutes or so between coats. But the problem is occurring early on, so I should force myself to go and do something else instead of standing there with the pad in my hand counting to 10.

Thanks for the help.

Just seems strange to me that it is only happening on one board which would suggest to me that your technique and mixture is correct otherwise your other boards would be effected the same way. Personally I think its something in that particular board causing the problem.