Reply from Sika last year when I contacted them about bonding brushed alloy to painted alloy.
Hello Julian,
Thank you for your call earlier and interest in the Sika product range.
I enclose the pre-treatment chart for use with the Sikaflex-500 series products. Once you have found the correct substrate I recommend selecting the pre-treatment option from the blue (level 2) column.
The pre treatment chart is based on best current knowledge only and it is always recommended to apply a bead using the chosen pre-treatment onto the substrate to check the bond is good before using on the installation.
Always read the PDS’s, SDS’s the pre treatment guide before use since they contain important information and advice.
I enclose links where the PDS’s and SDS’s can be located.
https://gbr.sika.com/en/industry/transportation/pre-treatments.html
https://gbr.sika.com/en/industry/transportation/caravan-and-motorhomes.html
Pre-treatment-Chart-STP-Sikaflex-500-series-EN-CORP-05-2017-V6 (13).pdf