Originally Posted by mr4speed
Widetrack has a very good point here. When your blocking and a low spot is found, it usually does not just "go away" because the guide coat has been sanded off. Almost 10 times out of 10 it is usually still there, especially if you are using anything other than a hard flat block. Just remember that half the thickness of a strand of hair in a low spot WILL cause a ripple.
Correct...and so often I get the question of "I blocked that panel 10 times and it's still not straight, why"? Most times the person doing the blocking is putting to much pressure or weight onto the tool while attempting to block. Let the paper do the work, light almost effortless pressure will straighten a panel faster!
The other reason is that they concentrate on removing the high or low spot when they the entire panel needs attention. Example, lets say you have a low spot on a door, if you block only on that area to remove the low spot all your doing is making the low spot bigger, spreading the low spot out if you will and giving the illusion of straight. This will seem fine until the piece is painted and you look down the side of the vehicle checking out that beautiful reflection and you find you have a "house of mirrors" look.
When ever I work on a vehicle and the customer demands that "gotta be perfectly straight" finish, I walk the panel when blocking. Also check your work after blocking by first cleaning the blocked area, drying it and then wetting it down with a pre-paint cleaning solvent and check your reflection at about a 45 degree angle. Pick something in the background as a reference point and move your head back and forth, up and down focusing on what ever you picked in the background as your reference point. When it looks like a mirror without any warbles in your reflection you know your panel is straight.
Hope this helps.