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I removed the old plastic wall tiles and this thin layer of brownish adhesive is what's left. I heated it with a hair dryer and it softened it enough to scrape off with a putty knife. Will I have to scrape all the walls or can I just start applying the thinset I'm using to adhere my ceramic tiles? Thanks

It might work but I wouldn't guarantee it for a customer! The plastic tiles were probably applied with a mastic - which explains why it is easily softened - and that may not be solid enough for thinset.

Hello Mountainbeach,
At the very least, you should scrape the residual adhesive from the wallboard, along with any loose paint that wants to come with it. If this is merely a cosmetic wall, and not a wet location (shower, tub surround, etc) then you can install new ceramic over the existing surface as long as it's solid and free of grease or other contaminates.
If I were presented with a situation like this by a paying client, I would remove the existing wallboard and install an appropriate underlayment to reduce mess and cost to the client.
Keep in mind also that much of these historic plastic tiles were installed during the heyday of asbestos-laden adhesives. Take EVERY precaution to prevent exposure to yourself AND the rest of the household just in case you are dealing with asbestos.
SERIOUSLY!
Best of luck,
Shaughnn

Mastic has a service life. Some of it is good for about 15 years, some longer. Thinset doesn't have the same technical limitation.

The old mastic is probably getting close to its age limit. Makes no sense to leave it in place, no matter how good the thinset you use over it.

If the subwalls are plaster and lath, you're stuck with using the hair dryer or heat gun, scraping the mastic off. If the subwalls are drywall, it's going to be far less time and effort to remove and replace the drywall. You're going to end up patching after scraping off the mastic anyway.

I'd R&R the drywall, if it were my job.

Regards.

The short version is NO! Looks to me your on plaster/mud walls. Do what you were doing with the hairdryer. a heat gun may work better. Soften it and scape it off. You can back trowel the walls with thin set after you have them clean. Let that dry and your ready to tile again.