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  #1  
Old 07-14-2022, 10:08 AM
georgemohr georgemohr is offline
 
Join Date: Aug 2021
Location: Ewing Township
Posts: 97
Default Primer report: SEM self-etching primer

Hi team RV,

As a new builder, one of the scariest decisions we make is our solution to the "primer wars". I want to report my results for our approach, which TLDR; turned out great.

Lets assume you have decided to prime. You are now confronted with the following challenges:
  • What equipment and facilities will I need access to in order to apply primer?
  • How toxic is this stuff?
  • How durable is this stuff?
  • How quickly does it dry to a 'handling' state?
  • How will I integrate priming into my construction workflow
  • How hard is it to source this stuff?

After some experimentation with other products, my partner and I settled on this product.

https://www.semproducts.com/product/...-primer/primer

Here are the advantages vs other primers:
  • No gun to setup, calibrate, clean, fuss with (but you can shoot it through a gun if you insist)
  • No need to mix or thin, which to me is an occult skill akin to witchcraft
  • Very quick and convenient to turn around a primed part in small batches
  • Ability to partially prime, or touch-up prime a part
  • VERY durable finish
  • Extremely easy to apply
  • Fast handling time (10 min or so!)
  • Can be sprayed with a 3M respirator safely
  • Available in cans for brush application
  • Available for purchase almost anywhere, Amazon, Walmart, etc.
  • Two coats are just translucent enough to still see all of your markings under the primer (this is not a small thing!)

The negatives are limited to cost. The spray cans can be had for about $20 as of this writing. You will need a lot of them

The best part of this approach is convenience. You will discover that priming is not a monolithic step comfortably stuck in the middle of a component build. It is instead better understood as an incremental step in the preparation of a part or set of parts. There will be times when you'd love to prime just one or two items, but have a ton more work to do before you can arrange a big batch. Sometimes you might want to prime just one area of a full part. Are you really going to go mix a batch, prep the gun, clear out the spray facility, shoot it, clean the gun, clean the booth, blah blah? Primer convenience is really important!

Second, and this is pure personal choice, we decided to spray all internals (ribs, spars, stringers, etc.) but only prime skins on the faying surfaces. And a really nice way of applying the primer to the skins is a disposable foam brush. SEM comes in both forms, so it's perfect here.

Our process is:
  • Prep parts by drilling, deburring, fluting, dimpling etc. to the greatest extent possible before riveting anything together
  • Scuff the parts with scotchbright or sandpaper. You can skip this step and still get an acceptable result for internal parts but go through effort and scuff for greater adhesion
  • Wash the parts in dawn and water, and let THOROUGHLY dry
  • Cart the parts outside to a cheap plastic table
  • Shoot (or brush) two coats on all surfaces about 10 min apart
  • Let cure, if possible overnight
  • Proceed to rivet

I've attached a couple of photos to show our results. Obviously I can't provide any long term data on durability or corrosion prevention, but I have faith based on my testing that this primer is really adhered well and should meet our needs.

Hope this helps someone stuck in analysis paralysis like I was!

-George
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Last edited by georgemohr : 07-14-2022 at 10:14 AM.
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  #2  
Old 07-14-2022, 10:31 AM
RYOUNG's Avatar
RYOUNG RYOUNG is offline
 
Join Date: Feb 2010
Location: NC
Posts: 91
Default SEM primer

Iíve also had good results using this primer when I chose not to setup for spraying with two-part epoxy.
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  #3  
Old 07-14-2022, 11:30 AM
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Low Pass Low Pass is offline
 
Join Date: May 2006
Location: Houston
Posts: 2,164
Default

Any comments on the durability of this product as compared to epoxy? I like the simplicity. But the DP50 I used on my first project was amazingly tough.
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  #4  
Old 07-14-2022, 12:42 PM
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agirard7a agirard7a is offline
 
Join Date: Aug 2009
Location: Newport, RI
Posts: 771
Default SEM

Quote:
Originally Posted by Low Pass View Post
Any comments on the durability of this product as compared to epoxy? I like the simplicity. But the DP50 I used on my first project was amazingly tough.
The product is great and adheres well to a clean, scuffed surface.
My interior finish paint is SEM gray primer and it has held up very well.
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  #5  
Old 07-14-2022, 12:45 PM
RYOUNG's Avatar
RYOUNG RYOUNG is offline
 
Join Date: Feb 2010
Location: NC
Posts: 91
Default

Quote:
Originally Posted by Low Pass View Post
Any comments on the durability of this product as compared to epoxy? I like the simplicity. But the DP50 I used on my first project was amazingly tough.
I also use DP50 two-part epoxy. I believe DP50 is more durable than SEM self-etching primer but in areas that arenít exposed to a lot of abuse I think SEM is totally suitable and I donít hesitate to use it.
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  #6  
Old 07-23-2022, 07:47 AM
imrah6 imrah6 is offline
 
Join Date: Sep 2014
Location: Lynnville, Tennessee
Posts: 29
Default Just about to start my RV-10 Empennage

It is my desire to prime all internal surfaces that will not see the light of finish painting by a professional.

Vans say they use Sherwin Williams #P60G2on all their quickbuild parts so I was figuring to use that. I do see it has to be applied with a spray gun. I'm not worried about that particularly.

Is the primer you are referring to the Marhyde self-etching ?

Do you remove all the plastic from the part before priming or only that on the side you are priming ?

Apart from the fact it may not look cool, is there a reason to not prime some parts with the SEM and others with the P60 ?

If it is a self-etching primer, why do you need to scuff the part ?

I'll take suggestions from anyone
Thanks
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I should go back to work. Even as an entrepreneur I never was as busy as I am now that I am "retired"
First time builder but hope it won't be the last !

RV-10 Empennage received mid July 2022
Workshop almost ready July 2022
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  #7  
Old 07-23-2022, 07:34 PM
ravenstar ravenstar is offline
 
Join Date: May 2021
Location: New Hampshire
Posts: 103
Default

Quote:
Originally Posted by imrah6 View Post

If it is a self-etching primer, why do you need to scuff the part ?
I'm not a paint expert as is obvious to anyone looking at my work so far, but I can say from my experience that you definitely need to clean and scuff the part adequately when using SEM. I find it bonds quite well if you do this correctly, but I'll relate what happened to me when I didn't (this is therapy for an embarassing error).

One of the opposing faces of my F-1211A fuselage bulkhead and doubler suffered a rather deep scratch in transit that I didn't notice until I stripped the blue plastic prior to assembly. I suspected it penetrated the alclad surface, so I sanded it out, cleaned both opposing faces and primed both with several coats of SEM primer. It worked really well, just as advertised. After riveting them together I figured it'd look odd with some of the primer showing, so I decided to clean the outer surfaces and prime them. I didn't rough up the surfaces mostly out of laziness as it wasn't terribly easy to get around the rivets, but I did clean well so that water didn't bead up. Sprayed the part, let it sit for a couple of days and eventually installed it into the tail cone. I closed the cone up, and then noticed the primer had started peeling and separating from the outer surfaces. It just didn't bond. Well, it's too late now to go back into the tail cone, rough it up and clean it properly and re-apply the primer. It looks like a real amateur job. (It is, but still.)

I think roughing up the surface does several things: It removes any existing oxide which can impair bonding, it helps remove debris making the final washing more effective, and it greatly increases the surface area the primer can interact with and bond to.

It really was very easy technically to rough up the surfaces I was going to prime (at least before rivetting). It's just hard to fight the mental resistance to roughing up a nice smooth shiny surface, and I wish I had taken the extra few minutes to do it. We build for education, right? Lesson learned!
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