Sounds like you'll need to remove all the old seasoning and start over.  At least that's what I've gathered from doing a search online.  Maybe someone else has a better idea?

Personally, I would chemically strip and re-season the whole thing with the Sheryl Canter multistep flaxseed oil method.

RRP said:

HeavyG said:

Personally, I would chemically strip and re-season the whole thing with the Sheryl Canter multistep flaxseed oil method.

Thanks - here's a link. 
https://www.cooksillustrated.com/how_tos/5820-the-ultimate-way-to-season-cast-iron

BTW I will be seasoning my new baking steel this week using this flaxseed method due to the suggested durability.

Or one could go straight to the horses mouth - http://sherylcanter.com/wordpress/2010/01/a-science-based-technique-for-seasoning-cast-iron/

lots of interesting info about the why's of her process and further interesting discussion in the comments.

If it was me I'd start out with a fresh slate so to speak. 
https://youtu.be/j6Tz3HnnCFs

RRP said:

SGH said:

@RRP
Im not a cast iron expert, but when mine gets to where they won't hold seasoning for whatever reason, I sand blast them. Sure you have to reseason, but it's worth it in my book. Sand blasting works miricales on cast iron without removing metal thickness. 

Thanks but none of my air compressors are powerful enough to use a sand blaster attachment and hiring it done isn't cost effective.

If you know anyone who has a connection at an electrical motor repair shop they generally have a chemical strip bath and sand blaster. The one I use will usually do it for a nominal charge, around $5 or free if you ask nicely.

Ron, it may be fond that has carbonized with use from not being fully removed.  

You will have to strip, but I wouldn't sand, or sandblast.  

A shot in the dark here, but try spraying some oven cleaner on a rag and wipe the the areas affected, let it sit.  May remove enough of the carbon fond without getting down to iron.  Or, lightly spay just the bottom, place in a garbage bag for several days.  You could mask the sides real quick so that overspray doesn't hit them, removing after spraying.  Then wash thoroughly, reseason the bottom with grapeseed oil (avoid flaxseed) via the stovetop on high.  

There are many schools of thought regarding which fat to use to season cast iron.

Here's a page that may be of interest - http://www.castironcollector.com/seasoning.php

I couldn't care less about the collector or display aspects of all this.

Ultimately the real seasoning of cast iron has to come from many many months (years?) of actual usage/cooking.

I like the flaxseed approach for initial seasoning. I've not had any problems with flaking or chipping. My suspicion is that some folks that do have not done a good prep job or take some other shortcut and that impacts the ability of the finish to adhere as best it can.

The beauty of all this is that stripping and re-seasoning is no big deal and one can always take a mulligan if they are not happy with one approach.

Try doing a bunch of deep frying in it. 

I reseasoned our very old pan that had been abused on our BGE (forgot to take off of grill and burned on beef fat from a sear) using flaxseed oil and following this 18 hour process.  It was a long process but worthwhile.  Very happy with the results.  Cornbread perfect, so are sears.  

http://www.thekitchn.com/i-seasoned-my-cast-iron-pan-with-flaxseed-oil-and-heres-what-happened-224612


 

if its a baked on fond like focker mentioned you can heat the pan hot, then dump in a 1/4 cup hot water and work it with a spatula, the instant sizzling will lift it off

RRP said:

fishlessman said:

if its a baked on fond like focker mentioned you can heat the pan hot, then dump in a 1/4 cup hot water and work it with a spatula, the instant sizzling will lift it off

it's not baked on fond and the areas are smooth to the touch. Oil simply beads up and runs off these areas and that's why I was thinking of actually sanding the bottom with emery cloth to get "into" the metal by a minute layer.

whatever barrier is there will lift off with sizzling water.  i use lard, i believe lodge uses flaxseed and ive had problems with those preseasoned pans

if you got the pan even hotter and the water skittering hot it would have been even quicker but water works well, you want to see the water dancing in the pan

RRP said:

fishlessman said:

if you got the pan even hotter and the water skittering hot it would have been even quicker but water works well, you want to see the water dancing in the pan

I gathered that, but I figured ifs I got it too hot then I might mess up the rest of the seasoning so I toned it down by using more water and more time. But I do thank you for pointing me in the right direction!

i dont worry about the seasoning any more, i routinely deglaze the pan for graveys etc, this weekend seared up 8 pounds of beef chunks for chili, deglazed pan and added that to the chili pot,, sauteed some veggies to go in and wiped the pan clean with paper towel. i am one of those were the pan stays stovetop and never sees the sink

jimithing said:

I'm not a huge fan of chemical cleans on cast iron.  I don't like the thought of the chemicals soaking in and then leeching out during cooking.  If that's a newer cast iron then I'd say it's time to mechanically remove the seasoning.  Orbital sander would be my preference. 

If that's an antique or you don't like the idea of sanding cast iron then I'd say look into electrolysis. 

http://www.castironcollector.com/electrolysis.php

Or, to avoid chemicals you could use one of these:

http://i.imgur.com/J8P1FGZ.gifv#oo