Steel tubing internal rust treatment - frame saver?

Servicing the headset on my Dart I've noticed some light surface rust on the inside inside of the head tube and can see it has crept into the top tube. Exploring further I found a little in the BB shell and I'm assuming there will be some in the sea tube and chain stays. I believe the tubing is a combination of Air and Spirit.

The BB shell rust came off very easily with a wire brush and although the HT is a little more set in I think I can get the majority of it off. The seat, down and top tube will be harder to get to and the stays particularly tricky with a brush.

Unless someone tells me different I don't think I need to be too worried about it (should I be?!) but I would like to stay on top of it and not let it get worse. What frame saver applications do people recommend and does anyone have a link to some long wire pipe brushes for getting down the tubing? Watching a tutorial on YouTube it was noted to avoid bb threads with frame saver, any reason? Also I'm assuming it would be a good idea to remove my alloy headset?

Thanks.

I use Dinitrol ML because its a lot cheaper than JP Weigle.

Don’t see any reason for you to avoid bb threads or remove headset cups, it’s just an oil that forms a coating on the inside of the tubes. It can be a bit sticky but wouldn’t think it would gum things up or anything.

You can make a good tool for cleaning corrosion and stuff out the seat tube from a wire like a wire coat hanger and a scotch pad/wire wool/sand paper.

https://m.youtube.com/watch?v=xRIFqasCOSg

I use Dinitrol ML because its a lot cheaper than JP Weigle.

yeah i use the same, i dont see framesaver for sale much anywhere anyway

I use ACF50 which is usually used on engines etc
Even WD40 or similar will give it a good coating.

Or linseed oil is the choice of artisans

POR15 Metal Prep followed by Bilt Hamber S50 cavity wax for me.

Cheers all, I'll order some of the Dinitrol and make some brush tools so I can give it a good going over.

Today I settled for swilling motor oil through the tubing so I'm expecting performance enhancements like you used to see in Castrol GTX adverts.