How to know when you are done block sanding?

jcmeyer5

jcmeyer5

Having fun learning!
Real quick, this is a door that I stripped, epoxied, filled, epoxied, filled again, epoxied, sanded out nibs, then shot with 3 coats of Featherill G2. The Featherfill went on 3 weeks ago. Today I decided I wanted to sand it... so I did. Using 150 grit I block sanded the entire door. I used black Featherfill and it guidecoated itself well. I blocked until I had no Featherfill untouched. I was consistent across the panel. I broke through to primer in a couple spots, filler in others, and some metal in one tiny spot along the edge of the door.

I FEEL like this grit is done, and that I should switch to 180/220 to smooth things out for final primer. But this is my first go at it, and maybe I’m wrong? I really am not trying to rush it... just don’t know any better. How do I know when I am done blocking?
 
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Once you hit filler, there will need to be something sprayed over the panel before sealer. Either more G2, 2K, or full strength epoxy depending on the amount of fill you need. Usually when you are hitting different materials, that is a sign it is not quite "there" yet. Two coats of nice shiny epoxy at this point will give you an idea of what you have, and what the next step may need to be. Ideally, blocking stops when you start breaking through to the epoxy, before hitting metal.
 
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For me, there is no substitute for actual guide coat. Your door will not be ready for paint until it is final blocked with only one material exposed. Filler, 2k surfacer, and FeatherFill all sand a little differently and one will cut slightly quicker than the other making it impossible to have it perfectly level with more than one material being sanded. You can be sure you are ready when you can lightly sand off a light coat of guide coat evenly with very little pressure from a long firm sanding poard.

I know there are guys who swear they can feel a panel and tell if it is straight but guide coat is a must for me. It will show you any deep scratches, pin holes in the filler, as well as low spots. Using a different color primer as a guide coat is good for your initial rough in but is not what will work best for you on your final blocking.

FeatherFill and other polyester primers are so high build, it is easy to end up with too much material on your panel. Your goal is to have as little material as you can on the door but end up with it straight enough your final blocking will not cut through into the underlying material. I also like to shoot two wet coats of epoxy after I am done working a panel and block it one last time but even then I guide coat it.

Guide coat can make a blind man see.........:)

John ;)
 
Thank you John. I did have some low spots I filled. I will be doing a guide coat step. I’m not in a hurry. I have some spray chalk stuff here at work I will try on black.
 
its why I like to use different color primers, you end up getting shades in the low spots when you make it down to the mostly straight depth and still have some new colored primer areas where you knew it was low.
 
I did the same thing. My "storage" epoxy was black, then grey when I started bodywork, then bluish body filler, and black featherfill.
 
John Long said:
For me, there is no substitute for actual guide coat. Your door will not be ready for paint until it is final blocked with only one material exposed. Filler, 2k surfacer, and FeatherFill all sand a little differently and one will cut slightly quicker than the other making it impossible to have it perfectly level with more than one material being sanded. You can be sure you are ready when you can lightly sand off a light coat of guide coat evenly with very little pressure from a long firm sanding poard.

I know there are guys who swear they can feel a panel and tell if it is straight but guide coat is a must for me. It will show you any deep scratches, pin holes in the filler, as well as low spots. Using a different color primer as a guide coat is good for your initial rough in but is not what will work best for you on your final blocking.

FeatherFill and other polyester primers are so high build, it is easy to end up with too much material on your panel. Your goal is to have as little material as you can on the door but end up with it straight enough your final blocking will not cut through into the underlying material. I also like to shoot two wet coats of epoxy after I am done working a panel and block it one last time but even then I guide coat it.

Guide coat can make a blind man see.........:)

John ;)
Click to expand...

Excellent post John.:)
 
Guide-coated the door... looked good. Threw the last full strength coats of epoxy on it.
 

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Working on the matching fender now. I have some low spots. See photo... on the left near the body line. I did filler there... must have sanded too much off. I am under the impression that I continue to sand until either a) low spot goes away or b) I break through to metal..
 

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Right. I meant breaking through the Featherfill. That was what was in my head at least. Too bad my fingers aren’t attached to my brain
 
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