when block sanding or color sanding?

I was curious about this, some people recommend doing this. I was just curious the reason.

Why should you sand in one direction when block sanding or color sanding???

You said "some people recommend this"...

When it comes to taking advice from people over the internet, it's advisable to be careful of the "advice". On the other hand, anyone who DOESN'T recommend sanding in one direction should not be considered a good advice giver. If you don't believe that, try doing you sanding in a circular motion.

Ok, "Skip Cain" recommends the following:

"After the urethane primer is completely dry,(so as not to shrink any more) dust a light guidecoat over the primer, and break out the 400 grit and wetsand (by hand) the primer until it is as smooth as glass. Use a rubber sanding block and sand in one direction only. When all of the guidecoat is gone you're ready to spray the first coat of base."

It's not that I don't trust people, I'm just curious if there is a reason it needs to be done this way. I'm not going to color sand in circles on a freshly painted car.

I'm more interesed if there is a reason to only go in one direction, such as the paper getting full of filler or paint.

I block sand in a criss cross pattern, basically NE and SW, then NW and SE for nice flat panels. Yes I use a compass when I sand http://www.chevelles.com/forum/biggrin.gif. Color sanding I just go straight horizontally in one direction, only because if you go back and forth and get a piece of dirt or abrasive under the paper you will get that nice "zing" sound of your paint getting scratched. Going in one direction and constantly flooding the area with water reduces the chance of scratching the paint, or at least it does for me.

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Dave
70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
68/72 chevelles
55/57/61 chevies
TC member #493
Never forget Earnhardt!

I also block sand primer in a criss cross pattern. Typically, I also want to sand following the body lines. For example, on a curved surface I'd sand diagonally over the curve. If I sanded in line with the curve it'd be harder to perserve the shape. So, for block sanding, I guess I'd say I really usually go in 2 directions but sometimes only in 1 direction.

I haven't done much colour sanding but in line and taking great care is a must. The one direction sanding is also a good idea. Keep away from edges!

Peter

I am with you guys in that I usually do about 75% of the color sanding "with the grain". In other words with body lines. But have any of you tried using an orbital sander for color sanding? I do not color sand everyday so I am out of the loop of the new things. But when I was repping paint we also sold paper and supplies for colorsanding with the orbital sander, wet or dry. I did some demoing with it and found it to be a great way to color sand. Instead of leaving scratches in the clear it left a "haze" to buff out. It buffed much easier.

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1965 Buick Gran Sport Convertible
1965 Buick Skylark H/T
"Fan of most anything that moves human beings"

I used the orbital and dry paper on my 70, it really worked great! Very easy to buff.

That's what I was looking for. Thanks guys!

MARTINSR, I've seen the stuff to do that but to be quite frank I'd be scared to take a power sander to my car since I've never colour sanded a whole car before. All I have done so far is fixed some little spots in the finish.

I would guess the big gotcha with power colour sanding would be grinding a bit of dirt left on the surface into the paint and leaving scratches.

Peter

I wouldn't expect someone to power sand the clear without LOTS of experiance. It scared the living crap out of me the first time I did it. But I have to say it is really pretty forgiving. You put a soft,thick foam "innerface pad" on the disc and then the paper. So the innerface pad conforms to shapes and lines on the body pretty nice. It really was kinda hard to sand through. I think if you have a good feel for regular wet hand sanding, you would have no problem with the orbital sander.

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1965 Buick Gran Sport Convertible
1965 Buick Skylark H/T
"Fan of most anything that moves human beings"

DANIELSON sanda deck.....waxa on waxa off
He's giving you the advice I believe for the guide coat only here.This pattern of sanding is to reveal the high and low spots of body filler.I dont necessarily agree with WATER sanding a pourous material such as primer but like the gentleman before me said ne-sw/nw-se with a hand longboard will do the trick also.

I would do a search There has been a lot of discussion on paint.

I have read that you color sand in two 45 degree directions