As far as I understand they have no right to ask you to not underseal it after its been fixed.
Applying underseal after the test is fine.
Yes but it hasn't failed next year's test George! It's failed this year's and the tester needs to satisfy himself that the corrosion that he failed has been repaired correctly, like Eggman says, something he is unable to do if the weld is hidden.
You could very well get into a situation where your repair passes this year but by next year some of it has separated from the parent metal due to insufficient fusion, I've seen that myself. The tester is not allowed to overly pry the repair but iirc he is allowed to test the weld integrity to a certain extent with the flat end of his 'toffee hammer'.
Are you allowed to weld a front crossmember and suspension wishbones or is that a MOT failure? I’d like to add some strengthen to them and seen this done before but is it technically legal?
The severity of corrosion in highly stressed components, such as steering and suspension arms, rods and levers, can be assessed by lightly tapping or scraping with the corrosion assessment tool.
In places that cannot be reached by the corrosion assessment tool, an alternative blunt instrument may be used.
A highly stressed component should be rejected if corrosion has resulted in serious reduction in the overall thickness of the material or has caused a hole or split.
Welded repairs to highly stressed components are not normally acceptable, other than where the component is made up of sections that are welded together. To pass, the repair should appear to be as strong as the original design.
Are you allowed to weld a front crossmember and suspension wishbones or is that a MOT failure? I’d like to add some strengthen to them and seen this done before but is it technically legal?