Sorry,
Wrong Bob; I meant Bob "Captain Avgas" Barrow. What did you use to replace the powdercoat?
Paige
RV-8A
Hi Paige, this is what I did to ensure excellent coating adhesion and performance.
I had the canopy chemically immersion stripped to remove Vans coating. Most powdercoaters will not have this service so you may need to go to a company that specialises in chemically stripping powdercoated products. I recommend this process as opposed to having all Vans powdercoat blasted off (safer...better).
This will leave you with a Rust Grade A finish (Steel with millscale layer intact and very minor or no rusting).
Take the stripped canopy to a high quality powdercoater that has blasting facilities. Specify a Class 3 blast done with a 30/60 mesh garnet at LOW PRESSURE (approx 30 psi) or equivalent. This will produce a complete blast clean with consistent metal colour all over and no visible contaminants.
Specify a primer of Orica/Dulux Zincshield Powdercoating Primer. This is a high zinc containing primer specifically formulated for powdercoating applications over steel. It inhibits corrosion creep under the powdercoat by an anodic process while exhibiting excellent intercoat adhesion properties. It is typically applied at 80 micron film build.
Specify a compatible Orica/Dulux polyester powdercoat top coat such as Dulux Duralloy (Product code 272 Line) or equivalent which is suitable for use on steel in exterior applications.
Provide the powdercoater with a small sample of 4130 tube with a wall thickness the same or VERY similar to the canopy tube and get him to run it through all stages (including blasting) with the canopy. Similarity of wall thickness is important to ensure you do not get false results. You will use this tube sample for batch testing. Without destructive batch testing you can NEVER know that you have an adequate adhesion (because you're not going to destructively test the canopy itself).
Batch test the sample by scribing 10 lines at 2mm centres in one direction and then another 10 lines at 90 degrees at 2mm centres to form a cross hatch (ie forming lots of little squares with each square being 2mm x 2mm). Scribe the lines with a "Stanley" knife making sure that you are cutting right through the coating to the metal. Clean the cross hatch with methylated spirits, dry, and apply a fabric reinforced packaging tape over the squares. Leave enough spare tape to get a good grip to pull on. Let sit for 24 hours to allow the tape adhesive to get a good bond. Pull off as violently as you can. If no squares come off then you can be assured that the powdercoating adhesion is truly excellent and you have a coating sytem that will last the life of the plane. If you lose four or five squares it is still OK. If a third or more of the squares come off then something has gone wrong (REJECT).
Now you have a system that will never fail.
Of course you will need to sand the powdercoat as specified by Sika to ensure proper adhesion of the Sikaflex (and use the appropriate primer). I would also use the batch sample to build up a full joint profile including the powdercoat plus the sikaflex plus some acrylic sheet. Then you can test this small sample to be assured that the TOTAL system has adequate adhesion. HOWEVER I am at pains to emphasise that adhesion is only one part of the equation. The design of the sealant joint geometry is the BIG issue (and the one least understood).
The Sikaflex process is one whereby aircraft structural components are being chemically bonded. That is a demanding regime and it requires quality control at all stages. By definition quality control means CONTROL OF THE QUALITY of all crucial processes. In other words if you bond onto a powdercoated surface but you have no way of knowing the integrity of the powdercoat system then you have no way of knowing that the total system will not eventually fail. Under those circumstances you just live in hope that everything will hold together without ever really knowing.
Of course my advice on how to achieve a 100% reliable powdercoat system in no way signifies that I endorse the use of Sikaflex as a sole use product to bond on an aircraft canopy. As you will know I did not go down that path myself because I had specific technical reservations about long term performance.
In the long run I suspect that some builders will have considerably more success with the Sikaflex process than others (ie some canopies will last a lot longer than others) based purely on how much they understand the technologies involved and on their attitudes to quality control. At the moment everybody is just doing whatever takes their fancy and I doubt there are any 2 canopies exactly the same in terms of design and method. It is therefore logical that some will outperform others.
I note that Bob Brown stated in an earlier post that he sanded the powdercoat off completely in the area where he applied the Sikaflex (ie he bonded to bare steel). I would have grave reservations about the long term success of that approach. He has no passivation of the steel to prevent long term corrosion undercreep spreading invisibly under the Sikaflex and thus destroying the adhesion of the Sikaflex to the steel.
Hope I've been of some assistance.