Difficulty removing spark plugs - use penetrant?
 

I am trying to replace the plugs on my W210 and after following the great DIYs here, I can get the plug boots off.

However, the plugs are not easily coming out. I've turned them about 5-10 degrees ccw but they feel as tight as ever. I keep checking my wrench to be sure I am turning them ccw!!!

Do people use a squirt of penetrant on these typically to help them along?

I had difficulty getting my plugs off the first time. I just took my time and eventually got them off (i.e. don't put alot of force to snap off the plug). When I put the new plugs in I used anti-seeze on the threads and dielectric grease on the plug boots.

Since then I have read that using liquid heet on the plug threads will have the most difficult plug come out with ease. I havn't tried this but if you do please let me know how it works out.

Since you can turn them about 5-10°, I give hope to using the penetrant. Work the plug back and forth, turning the plug out a little more each time. The back and forth motion will work the penetrant into where it's needed and may save you.

Typically we don't use penetrant, but as Ray mentioned the threads are coated with an antiseize compound to help the next time around.

Another trick I heard is to warm up the engine (I haven't tried it). Aluminum expands more than steel under applied heat.

Well I got them all out (and didn't end up using penetrant). I think the previous owner used something on the plug threads which I understand is a bad idea.

New plugs in, all is good.

I had a similar issue with the glowplugs in my suburban. I used a u-joint and an air ratchet and they came out without incident. Glow plugs are more difficult as carbon builds on the tip and they are much thinner then spark plugs and snap off in the pre injection chamber.

as long as they are being swapped and the area is cleaned afterwards, penetrating oil or brake fluid would have been a good idea to help.
since plugs have washers you dont really have to get too tight