I was asked by a member to post some pictures of what guide coat is and how it is used.
When you see how quickly it reveals problem areas you will see why guide coating is essential to getting the car straight.

First let me bring you up to date as to what has already been done on the sections we will look at.
1. Stripped to bare metal and DA with 80 grit.
2. Coated with ********** Black epoxy. This will help us to "see" our sanding progress later on.
3. Lightly block sanded the epoxy with 150 grit and began working panels to get them straight with body hammer and then filler.
4. I used my hand to "feel" the panels for straightness rather than using the guide coat on the filler - this has its flaws as we will see.
5. Sprayed another coat of epoxy on everything to seal the filler work and sand through spots.
6. Sprayed two coats of G2 Poly Primer.

Now some pictures. Applying 3M dry guide coat to the deck filler panel. There were some problems evident that need filler work in this area.


You can see how much darker the guide coat makes the buff colored poly primer compared to where I have begun to block sand.

I try to use the longest sanding block that is feasible for the area I am working on. You need the block to fit the area but also provide the most surface area to keep things flat. One thing that I am finding is I like the Durablocks less and less for this type of work. I much prefer the rigid, wooden handled, aluminum board when it comes to getting things flat. The durablocks are great for contours and I love their round and teardrop shaped blocks. The soft pad is nice too.


Some lows spots are showing. Now what isn't so noticeable is the HIGH spot that is between these low spots. As I began to block sand the area the high spot became evident, however, it wasn't so high that I got to bare metal before I was able to remove the low areas.


The next picture shows the finished results. The high spot only reached the top coat of epoxy and never even got to the filler under it.


Now for the problem area. This quarter had been damaged and repaired previously but the lower portion of the repair had rusted completely through. I had to fabricate the support panel and complete lower section of the quarter panel all the way from the rear of the car to part way into the wheel well. As stated earlier I used the "feel" method when doing my filler work and this area seemed okay however the guide coat shows a different story.
Remember the color layers of the products on there. Bare metal - black epoxy primer - Marson Platinum filler (white) - black epoxy primer - buff poly primer and black guide coat.

I am using a wooden handled long board on this panel with 150 grit.
In the picture you can see I have sanded through 3 layers and have gotten into the filler. Notice the low spot between the two high spots that I was trying to remove. When block sanding you have to resist the temptation to tilt or tip the block so as to get to these low areas. You can also see how pronounced the guide coat makes these problem areas look. There isn't much filler on there and the next layer will be black primer then bare metal.


This area required more filler work to get it right. This time I used the guide coat on the filler as well. to get everything straight.
I sprayed the car with another coat of reduced epoxy last night. It will get two coats of 2k primer to day and another guide coat and block sanding session this time with 400 grit. 99% of the car came out great on the first guide coat session however you can see by the pictures that quarter would have been waving at me when I applied the clear coat.

Here's the finished panel:


Deck lid with fresh clear coat (********** MS Clear):


As always if any of the pros have tips or advice to add it is welcomed.

Great post, thank you for the education!

These kind of posts makes me wish I had known about this forum when I started my project.

Very informative, thanks!

these posts are great...however they make me cringe thinking of all the work/time to get everything straight. i was thinking i would have the car almost done in a weekend...now not so much

Autobody 101
You hear painters mention guide coat all the time, but to learn what it means, you have to visit Autobody 101
Great job, Jim

jim, great post. this would make a good for a sticky. i just got done with the forward panel of the trunk on a 67 buick convert it was kinked badly from someone setting on it what a nightmare for a black paint job

What kind of block would you use on a curve surface like a fender well? Would it have to be flexible? I see how the guide coat works on a flat surface with a long board but is still a process you use on curve surface? Also I just purchased aepoxy and 2k primer from ********** but I got the same color should I get another kind of primer and different color to help with guide coating thanks

mj4444 wrote:What kind of block would you use on a curve surface like a fender well? Would it have to be flexible? I see how the guide coat works on a flat surface with a long board but is still a process you use on curve surface? Also I just purchased aepoxy and 2k primer from ********** but I got the same color should I get another kind of primer and different color to help with guide coating thanks

For any concave or convex type surfaces I like to use the Round Durablock. http://www.google.com/products/catalog? ... CFAQ8wIwAA

I like to use Black epoxy and then gray or buff 2k over it but you can use the same color just block sand to remove your guide coat each time.

If you spray epoxy and let it set for 24 hours you can lightly block sand it and your low spots will show up without any guide coat. Apply your filler right over the epoxy (no need to sand if done the next day) and when your repair is done spot prime the repairs with more epoxy. Wait an hour and then shoot your 2k and start your block sanding when dry.

I recently sprayed high build primer and thought I did a good job block sanding. I put down some more primer and this time I used a guide coat and found out how impossible it is to see or feel low spots. This would make a great sticky. It sounds obvious but until I tried sanding both ways I had no idea how big the difference is.
Thanks.

so i take it that if you repair an area with body filler and sand it smooth you do not want to reach any bare metal? or is that you dont want to reach the bare metal after applying the primer? i am currently working the body filler and keep getting to bare metal. im using a foam block sander and have the feeling that its too soft not really getting the area flat rather forming around the high spot if that makes sense. im working on a rounded area of the rear quarter panel of my miata. thanks all.