After researching online a bit, it seems apparent that this is a common problem with the Focus in MK1.

I have a rusty hole in mine but I've sourced a secondhand one.

While or after putting the replacement on, can I do anything to prevent this happening again?

Is it a case of putting some protective paste on it, in which case what would that be or best?

Would it be worth painting the internal part with Hammerite first then applying the paste?

Thanks very much and hope you can help.

Paste?? You simply need a coat of Waxoyl

I used to make a thick sicky mixture of old engine oil + grease + underseal and spread it on using an old paintbrush. It attracted dirt but it stayed on for years in places that didn't get direct water spray from the road. It never dries and it prevents air and water getting to the metal so it never rusts.  

Or Lanoguard  (I've got the kit and the grease is good plus the fluid sprays on easier than Waxoyl which needs to be kept warm and diluted with White Spirit etc.
It's expensive and it isn't on yet so no idea of longevity but if it has a liner it may be worthwhile.

https://www.lanoguard.co.uk/pages/faq

Not withstanding the good suggestions above.

If the existing wing has lasted 20 years and the replacement perhaps only lasts another 10 years, do you think the car will still be on the road when it's 30 years old ?

10 hours ago, unofix said:

...do you think the car will still be on the road when it's 30 years old ?

Well, that's what I'm planning for mine 🤣

1 hour ago, StephenFord said:

Well, that's what I'm planning for mine 🤣

Now, how did that happen? Must have been a real belly laugh - 2?

@TomsFocus I note you are confused by my last comment. It's simply that the post shows '2' reactions, when there was only '1' LOL

6 minutes ago, StephenFord said:

@TomsFocus I note you are confused by my last comment. It's simply that the post shows '2' reactions, when there was only '1' LOL

I'm confused by the two Unofix's, same as you are!  

Thought he may have a second account on a different device but all the details are exactly the same so both from the same account.  I've never seen that happen before lol!

1 minute ago, TomsFocus said:

... I've never seen that happen before lol!

Me neither, that's why I thought it noteworthy...

22 hours ago, pcaouolte said:

I used to make a thick sicky mixture of old engine oil + grease + underseal and spread it on using an old paintbrush. It attracted dirt but it stayed on for years in places that didn't get direct water spray from the road. It never dries and it prevents air and water getting to the metal so it never rusts.  

I remember that one from many years ago when O rode a motorbike - the oily rustproofing (along with the grease pump on lorry springs)  made a lovely skid pan ...