How to prep under-body/chassis after sand blast??

Discussion in 'Color is everything!' started by Paul_MN guy, Sep 10, 2022.

  1. Paul_MN guy

    Paul_MN guy Member

    Just got done sand blasting the rear half under body (floor pan, trunk pan, frame rails, inner fenders....ect) of my 69. Curious now as to how I should prep all my surfaces prior to spraying epoxy primer? Would an air gun be sufficient in getting all the loose abrasive & dust off everything or dare I introduce a pressure washer into the mix to blast everything down considering everything is bare metal?? The media I used for blasting was Black Diamond coal slag and the car is located in a garage in MN. 5 day forecast is lows of 55 with highs of 75, no rain in sight. All advise is appreciated!
    68436596717__9FA3C19B-A897-4707-96A7-5C74445DA9DF.JPG
     
  2. hugger

    hugger Well-Known Member

    What sand did you use?
     
  3. hugger

    hugger Well-Known Member

    If you used extra fine which I hope you did,...you can just blow it really well then go straight to epoxy and paint,...if you used coarse you got your work cut out for you,...as you're now gonna need to sand the surface with 80 to 120 then epoxy ,...or you can epoxy let cure then sand then re epoxy
     
  4. hugger

    hugger Well-Known Member

    If you want you can flush the surface with brake kleen but it's not a "must"
     
  5. hugger

    hugger Well-Known Member

    Ight so I was driving and didn't read your post completely,....that's the absolute worst possible media to use,..my suggestion,...get some extra fibe from your local brick supplier,..re blast it then you can go straight to epoxy and paint
     
  6. hugger

    hugger Well-Known Member

    You done this on your back?!

    If your not after a "correct " appearance and want to use what the surface you have,...I suggest Epoxy 2 even coats let cure for 2hrs then spray Upol or "Dominator" bed coating it lays with a nice texture and will sound deaden along with protrect and seal off small gaps in the sheet metal,..prolonging life
     
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  7. Paul_MN guy

    Paul_MN guy Member

    @hugger
    1. Yes, I did do it on my back. It sucked
    2. I rented the blaster Pot and tow behind legit air compressor so they're already returned and I aint renting it again.
    3. The grit of Black Diamond abrasive I used was the finest it came in labeled "fine (30/60)"
    4. Main goal of this was not to achieve a "correct" appearance but rather eliminate rust and prolong the life of the steel while then being able to work on my car without rust falling in my eye.
    5. I did the front half of the car last summer (see pic) but prepped the even harder way via angle grinder and 39487503498 wire wheels, flat discs, paint stripper wheels...ect. Never again I said thus renting of the blaster for the rear half.
    6. I ended up doing epoxy primer, black base coat + satin clear top coat on the front half last summer and still have leftover paint from that so I plan to do the same for the rear half of the car. I was considering doing some sort of bed coating in the inner fenders as it would seem that surface would take the brunt of abuse from flinging pebbles and such..... Cars a convertible and will have a 455 in it so I'm not too concerned with trying to quiet the cabin....
    So from what I gather taking my time with an air gun to blow down all surfaces should be adequate in prepping the surface for epoxy primer? My fear was that the fine dust particles would stay lodged in the now mico-abraded post-blasted surface and would cause issues with the epoxy adhering properly.
     

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    Dano likes this.
  8. wkillgs

    wkillgs Gold Level Contributor

    I like to go over the blasted surface with a DA sander with 80 grit. Get all surfaces you can get to. It smooths out the surface and also makes it easier to wipe down.
    Scotchbrite works ok too....scrub while blowing with air to help dislodge any blast particles in the surface. I'll smooth it out a bit too, making wipedowns less likely to trap lint.
    Wipe down with wax and grease remover, you may end up with lint on the surface. Go back over it with scotchbrite while blowing with air, then paint.

    Just blow everything out with air. A shop vac at one end on the gas tank support and blowing air into the other end will help get all the grit out.
    I wouldn't wash with water, it'll take too long for trapped water to evaporate. A metal conditioner with phosphoric acid before primer is a good idea if there is still rust on the surface, but not absolutely necessary.

    I blasted the underside of one of my 66's back in '04. The body was spaced off of the frame with 4x4's. It was quite a job, but the results were worth it.
    upload_2022-9-11_10-42-58.png
    upload_2022-9-11_10-43-33.png
    And after DA'ing and scotchbriting:
    upload_2022-9-11_10-44-14.png
     
  9. 12lives

    12lives If it runs, it's done!

    My 2 cents - You do not want a smooth surface on the under side. Primer/paint/bed liner likes a texture to anchor to. You can sand after prime if you want a smoother appearance. I would use a strong vacuum cleaner with a brush first to get most of it off, then blow. Be careful that your air doesn't have a lot of moisture in it.
     
  10. DaWildcat

    DaWildcat Platinum Level Contributor

    Or compressor oil!

    Devon
     
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  11. srb

    srb Well-Known Member

    I wiped everything down with degreaser, then coated with 2k epoxy primer, then oversprayeble rubberized anti chip, and then a black 2k chassispaint.
     

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  12. hugger

    hugger Well-Known Member

    Agree,..I like the surface of fresh blast as is,..it gives a perfect mechanical bond. Only time I would sand it is if the media profiled the metal too deep
     
  13. Paul_MN guy

    Paul_MN guy Member

    Appreciate the feedback from everybody!

    The post-blast surface finish roughness was not conducive for wiping and there was this slight grime to the dust encapsulating everything on the chassis and in my garage for that matter. Due to this I didn't trust shop air along to clean the metal before primer. Yesterday evening I spent a few hours hitting everything with a combination of wire wheel, orb sander, 80grit hand paper, scotch pad to smooth the surface enough to allow proper wiping but also leave enough feature for the primer to bond with. Wednesday is set aside for final blow down, wipe & Paint.

    I often have to remind myself all the work I've done to this car has been performed in a 100yr old single car garage located in a Minneapolis alley way. It's a challenge at times thinking if I was only able to take the body off the frame........
    One day maybe I'll live in the country and have ample space to do things differently, until then though I'll power through with what I have and learn along the way.
     
  14. Paul_MN guy

    Paul_MN guy Member

    Glad to be done with sandblasting and painting on my back...... turned out OK for what it is and what I had to work with. On to more pleasurable things now like putting parts back on without rust falling in my eyes IMG_9994.JPG IMG_9997.JPG
     
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  15. 12lives

    12lives If it runs, it's done!

    Looks great!
     
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