Scumming on sandpaper

[Mark]

Spent the day sanding with the RO150 using Cristal and Brilliant but the paper seemed to be scumming a lot faster than I was expecting. I have the RO set on speed 6 and using it in aggressive mode not orbital. I've attached an image of the paper. Both Cristal and Brilliant are clogging and I'm suspecting that I've got it all set up too fast and too aggressive but I'm taking down peeling paint to bare wood and want to do it as fast as possible.

Advice?

 

[Roseland]

You, or the vacuum (assuming you are using one), may be pressing the sandpaper too hard into the work.

Festool recommend that when using their sanders you do not use full suction for this reason.

Andrew

 

[jvsteenb]

You say the paint is peeling - is it old paint or is it just peeling because there's a mismatch between the layers ?
Or is it perhaps hot enough to become plastic ? If so, then scrape it of, it'll take less time and expensive abrasives.
In my experience, if - for whatever reason - the paint is gummy, there's nothing to stop it from clogging op a dry piece of paper.

Just my ?0.02 .....

Regards,

Job

 

[John Langevin]

The "Corms" (or scumming as you call it) are basically the result of too much pressure and the resulting heat buildup on the pad which causes the paint to melt. I have found it best to pull-scrape any loose paint/dirt off first using various Bahco Carbide Scrapers; it is much faster and much cheaper than sanding.
Once you are down to well adhered paint you need to use the adjustments inherent in the Festool System to find the ideal balance of sanding pressure vs. limiting heat buildup. I think for starters you need to slow down the sander to say 4 or 5. Unless you are using less than 100 grit or so there is no advantage to using the aggressive mode. The vacuum speed setting is crucial in allowing the sander to just barely float over the surface. The ideal vacuum setting is negated if you insist on pressing down on the sander.
I also do my best to try to keep the abrasive clean, when conditions permit, I rub white school chalk on the pad to discourage sticking. I also use a brass shop brush and a broken X-Acto knife to pop the corms off. Let us know if any of these things work for you.

 

[Brice Burrell]

Not mentioned yet is keeping the sander moving. Sanding one spot for even a short period of time (15 seconds) can quickly overheat the area and melt the paint.

 

[AlexThePalex]

The "Corms" (or scumming as you call it) are basically the result of too much pressure and the resulting heat buildup on the pad which causes the paint to melt.

It doesn't necessarily have to be a heat effect. It can also happen when the paint is dirty so the grease in the dirt will sort of glue all dust particles together. That's why I always clean a surface with ammonia before I start to sand.

It can also happen when you sand water based acrylic paint. Acrylic paint doesn't react well to sanding, especially wen you use the lower grits.

And sometime there is just nothing you can do about it. Not all paint will be completely hardened out under all circumstances. Even if it is hard to the touch, it might still be somewhat of an emulsion instead of solid paint.

 

[jvsteenb]

There's a valid point. Grease and grime will very effectively clog up your paper as well, and may ruin the adherence of a fresh layer in the process.
I routinely clean paintlayers before sanding but I tend to forget that what I take for granted may not be obvious to others.

I don't know if you're near the coastline ( being from Honolulu, I guess you are ? ) but salt, (being extremely hygroscopic) will make your abrasives clog even faster.
For exterior paint: clean, rinse, allow to dry, sand. Don't leave out any of these steps, and sometimes an extra clean and rinse cycle may be needed.

I must admit I'm glad that nowadays there are some very good alternatives to ammonia though... I've always hated the stuff.

Regards,

Job

 

[MarkF]

Have you tried a very coarse abrasive?  You can't avoid this on some finishes even with a very coarse grit but I would try a P30-40 range and just let the abrasive do the work.  If you continue to have problems get a container of denatured alcohol or mineral spirits depending on the finish (I use an old Coolwhip container with a lid) and soak the abrasive for 30 minutes.  You'll be surprized how much of it comes off easily with an old toothbrush leaving a fresh surface.  Let it dry and march on.

 

[Mark]

Mahalo nui loa for all the advice! Using 40 grit Cristal which works wonders but due to the paint being waterbased and sometimes baking in the sun it could just cause the "corms" to form. It's pretty apparent when the sander begins to hop over the surface which usually means its time to clean or change the paper. Not using very much pressure as I'm used to letting the Rotex do its thing and since its T&G redwood I'm careful to not gouge the wood when it cuts through the paint. Dirty substrate could also be the problem as we've been going slow on this and we pressure washed the house a couple of weeks ago. Not too near the ocean so I don't think its salt residue that is causing the problem. One thing I do notice is the corms are forming in line with the holes in the abrasive. If you look a the picture you can see the corms forming in arcs. I'll try these suggestions and see what happens this week. I do have to say that I can't imagine how much harder this job would be without Festool backing me up!

 

[family]

A lot of the problem is the fact it's paint. I can remember when paint would get rock hard on a surface. The downfall to this was that when the wood expanded and contracted it would crack and scale off. But you could sand it because of it being so hard... you just had the dust to deal with. Later the development was that it would get hard but it also would chalk...supposeably the continuous chalking gave a fresh looking painted surface...even this was still sandable, you just had to deal with the dust. Today paint consist of acrylic mixtures...forgiving when the wood expands and contracts. Acrylics dries to a pliable plastic coating. Acrylics and plastic don't like heat. The friction/heat from sanding causes the paint to melt and become sticky. I haven't found a grit grade of sandpaper or combination of that I didn't get frustrated with this very problem, Corms. 

 

[Mark]

Loaded a 40 grit Cristal on the Rotex and set the speed to "5" and vac to mid range and it seemed to work better. The problem is that at such a high speed the paint heats up, melts and forms the boogers on the paper. Again, thanks for all the advice. Just a couple more walls and we're about done!