I am trying to remove the stickers/decals so I can repaint the bike.
I have tried scrapping, paint thinner, and xylene.
I also tried using a heat gun, but the process seems rather tedious.
Is there some method that is faster?
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I'd suggest stripping all the components off the bike (yes, ALL of them) then hit the frame lightly with a flap disk in a hand grinder. You want to key the paint, not necessarily remove it.
Keying provides a mechanical surface that is not flat. This will allow your new paint to form a three dimensional bond with the previous coat and will be more abrasion-resistant.
You also have to mask off every part that doesn't need paint. That's the bottom bracket shell and threads, and anything else that has threads. Your fork's bearing races must be paint free, both on the fork and in the head tube. Noone paints their seat post because tolerances are too fine and it will scratch up anyway, so remove that. There's no point painting the inside of the seat tube, but a little overspray's no big deal.
Your frame appears to be a Huffy, which is a low end bike so probably made of steel. So any scratches through the existing paint will show bare bright steel which will need a steel primer. Sometimes this is called an etching primer, which chemically does a similar thing to the sanding process. Both is a great idea. Don't wait - you should be spraying the primer within an hour of finishing the sanding. Don't give the rust monster a chance to get stuck in.
If your bike frame is aluminium/aluminum then you need a primer for AL and not FE.
Then when the primer is hardened, a light wet-sand over the whole frame to remove any flecks and provide the next keying surface, for your top coats to sink their teeth into. Use this to examine the frame for light spots and missed areas. There's a lot of surface area to your bike - more than one might think.
Depending on your paint and its thickness, density, and grippiness, you probably want to do a minimum of two top coats. Some of the 80s bikes had 12~20 coats of paint total.
If you're adding new decals, then put them on top of the last colour coat after light sanding and wipe down, and do at least one clear-coat over that. If no decals, do the top coat anyway. This provides the first line of defense against dings, whangs and scratches.
You can choose a matt or satin or gloss finish at all layers, but the clear coats are on the outside and give the most finish-effect.
If all this sounds like too much work, and your plan is just to blast it with a rattle can with no preparation, then go ahead. Its your bike, do what you want with it. But expect it to look like a rattlecan job and to flake off pretty quick.