bainzy
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Inspired by the magazine articles comparing various products for rust proofing performance, I thought I'd have a bit of fun and try a test with some of the paints I'd bought and used over the years.
I've always been curious as to what happens under the panels over time when resistance spot welding. The usual anecdotal comments people give range from "bare steel only, weld-through primer will burn off anyway and hinder conductivity" to "zincy gooey mess can only be a good thing". At first, I had this vision in my head of these seams being filled with powdery soot, or some sort of liquid paste.
The automotive industry issues repair guidance and some manufacturers have shied away from specifying weld through primer, some still insist on its use. None appear to advocate copper based weld through primer, only zinc, with 3M pointing out that copper is lower down the reactivity scale than carbon steel. This means unlike zinc which becomes sacrificial to the steel, the steel becomes sacrificial to the copper. Eek! So for this test I will only be using zinc based products.
The Method
For the test I'm going to be using my Clarke CSW13T spot welder, and 1mm+1mm mild steel. The settings on the spot welder will be as per the thickness, and I will be using it's nifty square wave pulse weld feature. This mode burns off the protective coating for a better weld, the importance of which becomes apparent later on.
The steel will be cut into similar sized strips about 25mm wide, 75mm long. Each piece will be wire brushed to remove the coating on the steel, then sanded to 80 grit to give a good key for the primers. It will be then dipped in phosphoric acid, rinsed, dried and then immediately a pair will be sprayed with the primers. Only 1 coat. One pair will not be sprayed and will be relying on the phosphoric acid coating for rust protection.
Paints / Primers used:
1) Bilt-Hamber Electrox. This isn't marketed as a weld through primer, but some people appear to use it as such. I was curious to include it mainly as it purports to create a dry film containing 90% zinc, which is a lot of zinc.
2) Bonda Rust Primer. This also isn't marketed as a weld through primer, but I have again heard it reported that it can be welded through. It is a non-porous alkyd resin, contains zinc and is a primer I have used a lot following recommendations on Triumph forums.
3) Frost Weld Thru Zinc Primer. The only one that's actually a proper weld through primer! I've again been using this a lot recently. I'm very interested to know how it performs against the above Bonda Rust Primer, because I've been using both on panels, with the Frost weld thru primer reserved for painting the seams.
Test 1 - spot weld the pair of steel pieces 4 times. Three welds with a 15mm spacing and the fourth with over 20mm spacing. Cleaning the electrodes between each weld.
Purpose: see how well each welds. Then separate the pieces with a spot weld drill bit and assess what's happened underneath!
Test 2 - store the steel strip pairs in a jar of salt water and see what happens. I'm expecting the mix of acid dipped steel, bare normal steel (from the spot weld drill holes) and primer on these strips exhibits rust at a different rate. The industry standard salt fog spray tests (which are much better than my rubbish unscientific test) apparently range from between 5% and 20% salt solution. I'm impatient so will go with 20% of our finest white label british table salt.
Purpose: see how the products compare when immersed in salty water, as they might be if it got inside the seams of my classic car.
Disclaimer
I highly doubt this test would stand up to any sort of scrutiny or repeatability. There are so many flaws with it, it really is hard to argue it is a fair product comparison. For example:
- I cut the steel in a hurry and the size of each strip isn't mm perfect, one of them even got a bit chewed in the steel cutter.
- It doesn't do anything to test how good the welds are beyond me seeing that it's made the weld or not
- The thickness of coatings isn't precise
- Only one of the products is sold for this purpose
- I have no idea what I'm doing
However as you'll see I think there's some positives for all of the sprays included in my DIY test.
The First Test
Each strip was painted up using a high tech mask of card and some parkside mole grip boxes to weigh it down
The finished strips, prior to cutting. Electrox at the top (dark grey), then Bonda Rust Primer (red), then Frost Weld Thru Zinc Primer (light grey), lastly phosphated steel at the bottom.
The other side of these was not painted. Having paint on the outside of a seam will hinder the resistance spot welder from starting the weld.
Each piece was then spot welded like so:
Until all had been done.
First test result
I'm pleased to say that all the products appeared to work well in this manner. I already knew the Frost weld thru primer would do the job well as I've been using it, but all 4 pairs of steel seemed to give really good, strong welds. There was no visual difference in the heat affected zone outside the panels, and they were all as tough as each other to separate. Sparking from the welds was about the same for all of them.
I think this is good news for anyone considering these.
The next step for the first test was drilling out the welds to reveal the inside.
1) Bilt Hamber Electrox. This had the most scorched appearance, though appears to be an intact coating where it exhibts the scorching. The two welds on the right seemed to lose a bit of coating but I'm not sure if that was just from when I separated the welds. This one seems the easiest primer to scratch through mishandling.
2) Bonda Rust Primer. I would say of all of them, this has the neatest appearance. The scorching is very faintly visible as a slightly lighter red area, but it's hard to tell. But, is that good or bad? Does scorching demonstrate conductivity, impurity etc?
3) Frost Weld Thru Zinc Primer. Scorching here seemed less strong, with a very slight bit of browning around the spots as well.
4) Unpainted steel. This exhibits the same scorching patterns but instead of a black mark, if you look closely the molten metal is still there as a very very thin, shiny silver layer.
All together:
So closing thoughts on test 1 are that these are all potentially decent products to use as a weld through primer. They all welded perfectly strong spot welds, and as you can see, it's not like resistance welding burns off all the paint like MIG welding does. These spot welds might be on a wider piece of steel than the average sill or wheel arch lip, but I don't think it's fair to say all the primer will get burnt off. The pulse weld feature may limit or exacerbate any burn, I guess you could repeat this test without it.
So onto test 2....
Interested to know your thoughts. Will test 2 work? Is it going to take me months to see any results, or could it be within days?
I've always been curious as to what happens under the panels over time when resistance spot welding. The usual anecdotal comments people give range from "bare steel only, weld-through primer will burn off anyway and hinder conductivity" to "zincy gooey mess can only be a good thing". At first, I had this vision in my head of these seams being filled with powdery soot, or some sort of liquid paste.
The automotive industry issues repair guidance and some manufacturers have shied away from specifying weld through primer, some still insist on its use. None appear to advocate copper based weld through primer, only zinc, with 3M pointing out that copper is lower down the reactivity scale than carbon steel. This means unlike zinc which becomes sacrificial to the steel, the steel becomes sacrificial to the copper. Eek! So for this test I will only be using zinc based products.
The Method
For the test I'm going to be using my Clarke CSW13T spot welder, and 1mm+1mm mild steel. The settings on the spot welder will be as per the thickness, and I will be using it's nifty square wave pulse weld feature. This mode burns off the protective coating for a better weld, the importance of which becomes apparent later on.
The steel will be cut into similar sized strips about 25mm wide, 75mm long. Each piece will be wire brushed to remove the coating on the steel, then sanded to 80 grit to give a good key for the primers. It will be then dipped in phosphoric acid, rinsed, dried and then immediately a pair will be sprayed with the primers. Only 1 coat. One pair will not be sprayed and will be relying on the phosphoric acid coating for rust protection.
Paints / Primers used:
1) Bilt-Hamber Electrox. This isn't marketed as a weld through primer, but some people appear to use it as such. I was curious to include it mainly as it purports to create a dry film containing 90% zinc, which is a lot of zinc.
2) Bonda Rust Primer. This also isn't marketed as a weld through primer, but I have again heard it reported that it can be welded through. It is a non-porous alkyd resin, contains zinc and is a primer I have used a lot following recommendations on Triumph forums.
3) Frost Weld Thru Zinc Primer. The only one that's actually a proper weld through primer! I've again been using this a lot recently. I'm very interested to know how it performs against the above Bonda Rust Primer, because I've been using both on panels, with the Frost weld thru primer reserved for painting the seams.
Test 1 - spot weld the pair of steel pieces 4 times. Three welds with a 15mm spacing and the fourth with over 20mm spacing. Cleaning the electrodes between each weld.
Purpose: see how well each welds. Then separate the pieces with a spot weld drill bit and assess what's happened underneath!
Test 2 - store the steel strip pairs in a jar of salt water and see what happens. I'm expecting the mix of acid dipped steel, bare normal steel (from the spot weld drill holes) and primer on these strips exhibits rust at a different rate. The industry standard salt fog spray tests (which are much better than my rubbish unscientific test) apparently range from between 5% and 20% salt solution. I'm impatient so will go with 20% of our finest white label british table salt.
Purpose: see how the products compare when immersed in salty water, as they might be if it got inside the seams of my classic car.
Disclaimer
I highly doubt this test would stand up to any sort of scrutiny or repeatability. There are so many flaws with it, it really is hard to argue it is a fair product comparison. For example:
- I cut the steel in a hurry and the size of each strip isn't mm perfect, one of them even got a bit chewed in the steel cutter.
- It doesn't do anything to test how good the welds are beyond me seeing that it's made the weld or not
- The thickness of coatings isn't precise
- Only one of the products is sold for this purpose
- I have no idea what I'm doing
However as you'll see I think there's some positives for all of the sprays included in my DIY test.
The First Test
Each strip was painted up using a high tech mask of card and some parkside mole grip boxes to weigh it down
The finished strips, prior to cutting. Electrox at the top (dark grey), then Bonda Rust Primer (red), then Frost Weld Thru Zinc Primer (light grey), lastly phosphated steel at the bottom.
The other side of these was not painted. Having paint on the outside of a seam will hinder the resistance spot welder from starting the weld.
Each piece was then spot welded like so:
Until all had been done.
First test result
I'm pleased to say that all the products appeared to work well in this manner. I already knew the Frost weld thru primer would do the job well as I've been using it, but all 4 pairs of steel seemed to give really good, strong welds. There was no visual difference in the heat affected zone outside the panels, and they were all as tough as each other to separate. Sparking from the welds was about the same for all of them.
I think this is good news for anyone considering these.
The next step for the first test was drilling out the welds to reveal the inside.
1) Bilt Hamber Electrox. This had the most scorched appearance, though appears to be an intact coating where it exhibts the scorching. The two welds on the right seemed to lose a bit of coating but I'm not sure if that was just from when I separated the welds. This one seems the easiest primer to scratch through mishandling.
2) Bonda Rust Primer. I would say of all of them, this has the neatest appearance. The scorching is very faintly visible as a slightly lighter red area, but it's hard to tell. But, is that good or bad? Does scorching demonstrate conductivity, impurity etc?
3) Frost Weld Thru Zinc Primer. Scorching here seemed less strong, with a very slight bit of browning around the spots as well.
4) Unpainted steel. This exhibits the same scorching patterns but instead of a black mark, if you look closely the molten metal is still there as a very very thin, shiny silver layer.
All together:
So closing thoughts on test 1 are that these are all potentially decent products to use as a weld through primer. They all welded perfectly strong spot welds, and as you can see, it's not like resistance welding burns off all the paint like MIG welding does. These spot welds might be on a wider piece of steel than the average sill or wheel arch lip, but I don't think it's fair to say all the primer will get burnt off. The pulse weld feature may limit or exacerbate any burn, I guess you could repeat this test without it.
So onto test 2....
Interested to know your thoughts. Will test 2 work? Is it going to take me months to see any results, or could it be within days?