You wrap the sand paper around a specially made block it keeps the paper flat so the putty and primer is sanded evenly. It is a must in smoothing panels out. They can be picked up just about anywhere, auto paint supply and auto part stores. I would recomend using a hard block. I only use the soft blocks on bumpers and curved panels.
Not to jump in, but won't those "square off" a radiused feature instead of maintaining a perfect curve? I'm a total body newbie, but it seems like those blocks are too hard to do, say, the top 1/2 of a MKII door. Perhaps the grit of the paper has more to do with that though?
Thats why i mentioned soft blocks. They are flexable for curved surfaces. I only use a hard block on flat panels. Really narrow areas i will sand by hand. The blocks are mainly for leveling putty and primer.
If you use your hands to sand flat panels you are applying pressure unevenly because all 4 fingers(thumbs usually hold but dont apply pressure)press down leaving 4 pressure points.. This cuts grooves or waves in the substrait. The block applies this pressure evenly giving a smoother finish. This isnt usually a problem for small surfaces.
I see. I didn't realize the differences were that great. Thank you for the info. I'm taking an autobody class this semester and it's something I'm really interested in because I can be anal and it's okay
See I told you I was anal. I edited this just to add one "e"...:shakeshea
whats the downside of using a soft block on flat panels? i have a solid rubber block that works well.. its got some flex, but not too much.. about the consistancy of a tire.. is that considered "hard"? its softer than the solid hard plastic block i have!
I would consider that a hard block. I consider a soft block as something you can fold in half. Hard blocks cut harder so less work on the first of putty or primer. On the second coat i go to a softer block which is less agressive. Then i use a light grit and a soft block for finishing.