Cork & rubber sanding blocks - are they needed

[Tony Clarke]

Does anyone know the real reason why a cork or rubber sanding block is advised rather than a solid piece of timber to wrap the abrasive paper round. I have heard many ideas but none seem to have a viable explanation?
The best I have heard relates to grit being absorbed into the cork and not leaving a small furrow along the surface of the timber. This must mean if the grit gets between the cork and abrasive paper!!

[Auld Bassoon]

I believe that cork or rubber sanding blocks are best used on curved (concave or convex) surfaces as they are less likely to lead to a series of 'flats'. I use soild timber quite often as an abrasive backing block for flat surfaces. I also have a small collection of small Lee Valley's sanding grips *see http://www.leevalley.com/wood/page.aspx?c=2&p=20183&cat=1,42500) which are also quite useful.

[durwood]

The idea of a block is to get a true surface when you sand. If you use your fingers or palm of the hand on a large flat surface you will get an uneven finish.

A flat block works the same as a plane does. it travels over the surface and the high spots come into contact with it first therefore they are sanded off this continues until all the surface of the paper over the block is in contact with the surface. You then have a true flat surface.

Cork is used as it is light in weight, rubber is used generally for wet surfaces as it is unaffected by the water but works dry its just heavier.

If you have a thin flexible block you can press it around a curved surface to level it if you use a hard block you must rub around the curve not along it as this will put flats on the surface. Eg: if you rubbed a cylinder length ways with a hard block you could end up with a multi sided pole instead of a round one. A concave surface needs a block which is round underneath ideally the shape of the curve, a flat block would only touch on each edge of the block.

Vehicle painters always use a hard block to level the primers and colour coats its the only way to get a mirror flat surface. To ensure they get a completely flat surface they spray a guide coat (a coat of a contrasting colour) on the surface, low spots show up as the surface is rubbed as the paint is in the low marks.

There is nothing to stop you using any thing to back the sanding paper as long as it spreads the load and levels the surface. Fingers, thumbs pieces of rubber hose, dowel, plywood, anything can be used if the shape is suitable.

When painting a large yankee station wagon I used a large block of wood a bit smaller than a complete sheet of sanding paper and used drawing pins to hold it onto the block so it could be used to level the repairs.

If I can't find my cork or rubber block I grab a piece of timber.