Help with Block Sanding Grits

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PostPosted: Wed Sep 08, 2021 10:38 pm
I am prepping a hood for my 89 Chevy Squarebody C30 Dually to paint SS black (or BC/CC not sure which yet).

The temps in my Pig Barn shop can reach 100 degrees easily this time of year with low humidity here in Northern California.

I stripped the hood by having it blasted top and bottom. Then I did some metal working to it. I couldn’t get to all of what I would have liked to have without removing the under hood braces. Which I did not want to do. I figured that I would end up screwing up more than I could fix.

I thought that I had the metal work down pretty well and that I wouldn’t need any filler since I was using HB primers (I was wrong).

The first coat I did in epoxy followed by some Evercoat 4:1 Superbuild.

90 Burb Hood (4).jpg

90 Burb Hood (5).jpg

90 Burb Hood (6).jpg
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PostPosted: Wed Sep 08, 2021 10:40 pm
I then proceeded to block it out with 180 grit at first. I then realized that my metal working wasn’t as good as I had hoped and switched over to 80 grit to flatten it, did some more metalworking and followed with 180 blocking. Still hadn’t used any filler up to this point.

I stopped at the 180 grit as I knew I was going to be shooting more HB primer.

90 Burb Hood (11).jpg

90 Burb Hood (12).jpg
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PostPosted: Wed Sep 08, 2021 10:42 pm
I used some old Tamco 53xx that I had and put it on the shaker, it seemed to have a lot of air bubbles in it but I figured the strainer would take care of that. I didn’t reduce it enough (I think) as I was using a 2.2 tip on an old StartingLine gun that I have.

Things went bad fast, the material came out with a million little bubbles – I think due to not reducing the 53xx HB primer.

I stopped, added some reducer and switched to my Air Gunza AZ3 with a big ol 3.0 tip. This gun worked better but the damage was done. So I just finished with what I had.

I could see that I still had a low spot that was going to need some filler anyways. I started blocking tonight with 80 grit. You can see the spot that needs filler in the yellow circle. The far side is looking better as I had spent more time metalworking that side. I had to quit for the evening as pizza was calling.

So now my questions:

• If I know that I am going to be shooting a HB primer (Tamco 53xx, HOK KD3000, Superbuild 4:1, FeatherFill G2) do I need to go any higher than 180 grit?

At the point that I am at now, I plan on adding a little filler to my low spot, then shooting some more of the Tamco 53xx (reduced this time!) out of a new AZ3 with a 1.8 that I just got. Then block sand that out.

• If I get a nice smooth finish with the HB (which I believe that I can), what grit do I start with for blocking?

• After the next HB, at what grit can I stop hand blocking and switch to a DA to get to 600?

• After the next HB, Would I be better off to stop sanding at 400 and start my Top Coat with a black sealer(reduced primer)?


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90 Burb Hood (15).jpg
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PostPosted: Thu Sep 09, 2021 9:30 am
One step that would have prevented this from happening, is to lightly block sand your epoxy primer. You would have seen these problem areas immediately and filler over epoxy is fine.
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PostPosted: Thu Sep 09, 2021 10:04 am
Be careful using that long block in your picture, if not used properly
you'll get flat spots on a curved surface, that's what usually happens.
I never use one over 12", and on more pronounced curves I go shorter.
You can still get it just as straight with a shorter block if you use long strokes
with a coarse paper like your 180 grit, even 150 works better.
We all do it differently but I apply high build right over the 150 scratches,
guide coat and sand with 600 till everything is smooth.
It's a big jump from the 150 to the 600 but it works for me, but I also allow
my primers to dry at least 2 days before blocking.
Sanding with 600 is to fine to straighten so a soft block works just as well
JC

(It's not custom painting-it's custom sanding)



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PostPosted: Thu Sep 09, 2021 11:51 am
:goodpost: also remember not to put to much force on the sanding block, especially on large flat surfaces let the paper cut and keep it fresh.
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PostPosted: Thu Sep 09, 2021 12:10 pm
Going by your pic it looks like the blaster was clueless and destroyed your hood, unless you had kids jumping on it. I'd be ringing the blaster's neck. Been blasting for decades and never warped a panel. I own a resto and custom paint shop and have only been doing this for four decades. I'd find another hood or skimcoat that hood and start blocking. I don't have the time for the latter in my shop. By the way, warpage from blasting has nothing to do with heat. It is a myth perpetuated by the uneducated.

I use the longest block I can given the area. Also have many custom made blocks.

Forget the BC/CC black. Shoot single stage. If you want added depth then cocktail the last coat or coats with clear.

I often prime with PPG CRE epoxy. High build, block sands easily and is in the commercial line which is less expensive than the automotive line. Never gets any mention on hobby sites. Virtually the same as PPG 2050. Call your local rep.

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PostPosted: Thu Sep 09, 2021 12:53 pm
JCCLARK wrote:Be careful using that long block in your picture, if not used properly
you'll get flat spots on a curved surface, that's what usually happens...

You can still get it just as straight with a shorter block if you use long strokes
with a coarse paper like your 180 grit, even 150 works better.
We all do it differently but I apply high build right over the 150 scratches,
guide coat and sand with 600 till everything is smooth.
It's a big jump from the 150 to the 600 but it works for me...
Sanding with 600 is to fine to straighten so a soft block works just as well


Thanks JC, just to clarify, you block with 150, shoot another coat of HB, then straight to 600

Or, block with 150, then go straight to 600?

And I am being pretty aware of using my long block and the direction that I use it in. I actually start with the shorter block, then finish off with the longer block in the area that it fits without too much bending pressure.
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PostPosted: Thu Sep 09, 2021 12:57 pm
ScottB wrote:Going by your pic it looks like the blaster was clueless and destroyed your hood, unless you had kids jumping on it. I'd be ringing the blaster's neck. Been blasting for decades and never warped a panel. I own a resto and custom paint shop and have only been doing this for four decades. I'd find another hood or skimcoat that hood and start blocking...


Scott, I am curious why you say that the blaster ruined my hood. I did have another "expert" blaster screw up a pretty rare 61 C10 cateye hood - my fault for trusting them.

I thought that this one came out pretty well.

Just wondering what picture shows it so bad as I am just a weekend part time rookie at this stuff.
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PostPosted: Thu Sep 09, 2021 2:07 pm
OldFatBald wrote:Thanks JC, just to clarify, you block with 150, shoot another coat of HB, then straight to 600.


Yes, and I usually spray 2 to 3 coats with a guide coat
before sanding with 600,
Filling 150 scratches you really need to give it a day or 2
to shrink down before sanding smooth.
JC

(It's not custom painting-it's custom sanding)
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