Save yourself a lot of hassle, take it down to your nearest small local garage and it will take 2 minutes with a Buzz Gun and an impact socket. Most would not charge you or say give me a couple of quid.

Use a better grade of penetrating oil, like PBlaster, Kroil, Liquid Wrench, etc. WD-40's only decent application is spraying on wet electrical contacts. Buy a 3/8ths drive impact driver and 6 point impact socket 14MM or 9/16ths. After a day of thorough soaking, set impact driver to reverse, and whale w/ 2 lb. hammer. Heat won't really help in this application, you need to heat the metal that the bolt is screwed into, and mower blade is probably in the way. If you have access to air compressor, an impact wrench (gun) would work better.

Usually this is not a rush job. Just because you put on WD40 does not mean it works instantly. Put lawn mower in the car and take to a car repair garage. These guys have air hammers or air wrenches to put on or take off nuts and bolts. Those toys will usually do it. Faster than you can say "uncle"

Sometimes you can free seized bolts by tightening them first, the idea being you only have to start them moving one way or another. but like everyone says take the mower to garage and ask them to have a go at the bolts.

Hi besides wd40 a can of plus gas would be the best solution painted on left over night it usually works. if not yes heat it with a blowlamp.

"Just in case"

try tightening a couple first,

wouldn't be the first time I've found out that they were left hand thread,

Plus tightening fasteners a little can often help with loosening.

So I recently bought a new 21" blade for my old Honda mower, and then discovered the bolts that hold the old blade on are literally seized with rust. I've cleaned up the bolts with a wire-brush angle grinder attachment and soaked the area in WD40. Using a socket wrench and a pipe for leverage over the wrench I still can't get them to budge, the only result I got was a cracked 14mm socket. I've also tried giving the wrench sharp hits with a hammer and don't seem to get anywhere with that either. I've heard about heating the area with a blow torch before trying and have yet to give that a shot. But I'm wondering if anyone else has any suggestions that might help get these two bolts off. Cheers