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Do Axminster clamps cause stains?

Some Axminster clamps I bought recently have a potential problem. The metal jaws have plastic shoes to soften the grip on the wood. This saves putting a pad between the jaw and the wood. It makes them easier to use because clamping with pads can be one of those jobs that need more hands than most people possess. The problem is that the shoes mark the wood with what looks like a grease stain. The plastic looks clean and dry, but keeps making these ‘stains’ as if something is seeping out of the plastic. Is that possible? My old clamps don’t do it. The Axminster rep says this has not been reported to them before, but he is looking into it and promised to get back to me. If the marks are oily or greasy, they could interfere with gluing and finishing.

Later I had another call from Axminster Tools about my clamp problem. Their theory is that the clamp pressure is burnishing the wood surface. I don’t think this is right so I did some tests.

I put one of my Axminster clamps on a piece of MDF with thin polythene under one of the plastic shoes. The pressure must have been the same on each side. The side with polythene is unmarked, the other has a very prominent stain.
I put on another clamp with a small MDF pad under one plastic shoe and nothing under the other. There is a heavy stain on the top of the pad, nothing under the pad and a heavy stain under the other shoe. The pad was the same size as the shoe so the pressure must have been the same on all three surfaces.
I think these results strongly indicate that something from the plastic shoe is staining the MDF.

I spoke to Axminster Tools again about my problem with their clamps. I had to call them again, although they had promised to call me, and found they had done nothing about the issue. Disappointing, as they are normally very good at customer care. This is an example of what the clamps do to a bit of MDF.

Stain on MDF
MDF marked by clamp
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