Anodising a bike frame can be difficult, even if it is new.
Knowing what grade of alloy, plus the grade of weld wire used is important.
The frame has to be clean etc.
This gives an idea of some of the issues with anodising :
https://www.saf.com/faqs/what-aluminum-alloys-are-recommended-for-anodizing/https://www.saf.com/training-guides/preparing-aluminum-for-finishing/"Aluminum Anodizing Guidelines
Always confirm the alloy to be sure it’s anodizing quality (AQ). Order all your anodizing metal from the same lot to reduce color variation caused by the aluminum’s metallurgical composition.
Die-castings are difficult to anodize because of the porous nature of the surface, even if the alloy is anodizing quality.
Because anodizing is only as good as the metal you use, it will not hide scratches or water stains.
Assemblies must have drainage holes to prevent solution entrapment. Top holes admit air, while bottom holes drain. Anodizing chemicals can leak through even the tightest weld joints over time and ruin finishes.
Assemblies must not include non-aluminum materials. Be sure to select the correct alloy welding rod — we recommend 5356. Never use 4043 since it will turn a smutty black in the anodizing process."
https://www.moss-metal-finishing.co.uk/finishes/anodising/"Not all aluminium alloys are created equal when it comes to anodising. Moss Metal Finishing’s guide outlines the suitability of different grades for both decorative and hard anodising:"
"Internal threads may require masking to maintain critical tolerances due to the increased thickness from anodising."