Any question answered...

Media blasting is the quickest/most-common way to strip ferrous metals.

There are a few variables in terms of blast medium, pressure, distance, application etc.
It's quite an agricultural process all-told and lots of industries utilise it... to that end, you will find many people who have a blaster in their workshops. You don't need a qualification or a certificate from the Local Authority to buy a blasting setup. That considered then, you'll find there is good media blasting and bad media blasting.

Taking it to someone who isn't experienced in bikes can be an unwise decision as they might not protect threads or appropriately bung any breather/drainage holes on your bike... this will almost certainly see you get shitty dust and grit in your work when you move it around!

In a worst case scenario, a bike can be destroyed; I've seen it happen. On a lugged bike, the lugs can have their crisp shorelines compromised for example.

If you go the blasting route, I'd recommend being clear with the person doing it for you, what you will expect of the job in terms of masking etc. There are people who will take a tenner off you to blast a bike in an hour. There are people who will bill you £200 and take a day to do the same thing. Spending £50-£90 is potentially a good indication of the blaster understanding the value of their work, and your substrate.

Most blasting processes will change the texture of the surface of the steel. Sometimes they will also remove steel. The goal is for the blast medium to be harder than the paint but softer than the steel... aluminium oxide is common and in an optimal circumstance you would prefer it to be fresh/not-recycled from another project as this used medium will have bits of whatever was blasted off the surface of the job before yours; if that's paint then no worries, but if its rusted metal and corroding bits of shit, then you should swerve it.

You'll usually want to prep the steel surface once it's been blasted. Abrading it with a 120 grit and red scotchbrite is a good bet. The scores and scratches from the abrasives will make sharp edged peaks and troughs into which the primers will flow and "tesselate" for the mechanical bond. The finish from blasting usually leaves a softer-edged, more uniform texture and this gives a less-effective 'key'.

I'd recommend you get all your finishing stuff sorted before it goes to blasting so that you can start as soon as it comes back and it spends as little time as possible in a raw state.

If you want to go the route of stripping it yourself chemically, this will give you the benefit of being able to maintain the surface of the steel to a higher standard but it will in turn mean you've got to deal with some nasty, nasty chemicals. In my daily work, I would use Starchem Synstrip on steel but I'd effectively have a full power-ranger hazmat suit on 'cause this stuff really will burn your skin. For a DIYer, work in a clean space, work from the bottom up, cover the bike, wrap it in tin foil, go to lunch, come back and then working top-down, use the scrunched up foil to gently scrape it all clean. Depending on the paint, you might not even need to abrade with the foil at all, the stuff will just peel right off on its own. In rare instances, you'll need a couple of rounds. As soon as you've stripped it, you'll need to abrade it and clean it and dry it to prevent rust. I usually only like to chemically strip non-ferrous metals as on occasion I'll need to use water to clean off or neutralise the stripper.

Unless you are a strict "don't pay someone to do something you can do yourself" type person, or if you plan to do a few projects, chemical stripping isn't the way to go really.

I tend to save a few frames and make a trip to the blaster once I've got a car full of projects. If you want to get your frameset to me, it can join my batch and I'll contact you once it's done, usually a couple of weeks later. It'll be £50. You can also ask for tips/tricks and I'll give you some abrasives so you don't have to buy a box of red scotch in order to use two pieces of it.

For a casual one, you could go to Purpose Powder Coating on the Old Kent Road and pay them cash to do it for you. Depending on how they feel, the've billed me anywhere from £90 - £120 a frame.

For a very pro job, I'd recommend RFI screening but this £90 - £150 plus shipping both ways to and from Stevenage (I think its Stevenage).