Blotchy spots are almost always caused by uneven grain texture and burl is the most prone to it.  From your area code, I assume you are turning big leaf maple. The straight grain BLM isn't too bad but the burl can be a bear.  More coats might help if you are using just an oil/ wax finish like DOC-101 or -110.  If it's a shellac containing finish, more coats probably won't help.  What happens is that the soft spots absorb the finish very quickly and as the shellac dried it makes a rough spot, not just a soft spot.

How do you fix it? 
For the current piece, I would sand it down to the surface and start over.  You are correct that a sanding sealer will fix the problem.  I use a sanding lubricant to push walnut oil into the wood.  By the time sanding is done, the surface washed and friction polished, the wood is sealed.  With burl, apply the first coat without the lathe turning.  Look for even coverage.  Keep applying by hand until you get the surface covered evenly.  Turn on the lathe and polish.  You should see an even coating and you are off and running.  If not, sand the spots again and keep applying finsih by hand.

If you don't use the sanding lubricant, a sanding sealer is a good idea, especially with burl.  Spray shellac is ok, but a 1 pound cut of shellac in a spray bottle is a better choice.  Spray the surface while the piece is slowly turning.  You will see when the surface is evenly covered.  Let it dry, cut back any rough spots with steel wool and start to apply your finish.  

Walnut oil is more likely to solidify than become rancid.  Rancidification is more common in unsaturated oils that don't polymerize, non-drying oils like canola and olive oil.  Not saying that it can't happen, but i have oil samples from three years ago that are fine.  The oil in a product like Pens Plus is even less likely to go bad because the oil layer is protected from the air by the shellac.

When a finish is spotty, it is almost always because the wood is taking finish up at a different rate.  The can be soft spots, especially along growth rings that will absorb the finish so you get an uneven top coat.  Even though the acacia is a very hard wood, it is very sensitive to this problem,  Because the HBFP is a very thin (high alcohol) finish it is also prone to this problem.  A sealing layer is the best way to address this problem.

The HBFP will evaporate very quickly then you apply it, there needs to be a good base layer.  The Walnut Finishing Oil (DOC-106) used as a sanding lubricant, produces not only a better surface but you get a sealed surface when you friction polish the WFO before applying the film finish.  

A sanding sealer can also help.  If I have both light and dark wood in a project I don't use the WFO (to keep the colors clear).  I use a 1# cut of shellac to seal the surface, Bulls Eye Clear Coat Shellac diluted 1:1 with alcohol.  Spray on a couple of coats and repeat your last grit of sandpaper.  Slow down the lathe to about 300 rpm for the first coat of HBFP to allow coverage without evaporation to get a good smooth base layer.  

I keep a spray bottle of this by the lathe for sealing but also for tear out.  Sometime stiffening the wood just a little bit will make the cut much cleaner and reduce sanding time.

Using the Walnut Finishing Oil (DOC-106) as a sanding lubricant does several important things.  First, it allows you to sand at a higher lathe speed without fear of heat checking and it produces a burnishing compound to polish the wood as you sand.  We forget that sandpaper is a cutting tool. Thirdly, it forces the oil into the wood to provide a deeper oil finish, stabilizing the surface of the wood.  This improves the quality of whatever film finish you apply. See the discussion of sanding lubricants under Extras and in the video section.

Shipping and shellac are my two biggest headaches.  All of the liquids must be shipped by ground.  West of the Rockies, $9.50 will pay for one bottle to be shipped by FedEx Smart Post or USPS. Anywhere else it will be $9.50 or more.  That's ok, most companies build some shipping into the price of a product, I just can't build in that much. Also, the more packages you ship, the better price you get.  I'm not Craft Supply here.   I prefer to ship by FedEx Ground or FedEx Smart Post (except to Alaska and Hawaii or PO Boxes).  It is a five to seven day tracked delivery to anywhere in the US. So why not ship everything by FedEX ground? That single bottle costs $13 to ship by FedEx. If the FedEx Ground is close to what you have been charged for shipping, I will send it FedEx Ground.  Especially to the east coast. UPS is always more expensive than FedEx.   I have started using FedEx Smart Post; FedEx gets it close and the USPS actually delivers the package. It is slower than FedEx Ground by a couple of days but less expensive.  I hope this will keep shipping costs down.

This is what I have learned from the medical literature. About 3 to 5% of the general population reports a nut allergy. Of those reporting "nut allergy", about 90% are allergic to peanuts, which are a legume and not a nut at all. Of the remaining 10%, 5% are allergic to cashews and the rest allergic to walnut, almonds and other nuts including filberts. Walnut allergy is elicited by the proteins in the nut, wood and leaves. The walnut oil I use is treated and filtered such that no protein is detectable by the most sensitive methods in my lab (mass spec and colorimetric). I can find no data on cross reactivity between nut allergens. I do not guarantee that there is not risk of allergy, but the risk is as small as I can make it. Moreover, since walnut oil is a drying oil, it is found in the wood as a solid, plastic-like material that is not going to be lost in a liquid form, making any protein residue unavailable for biological interaction.

When a finish is spotty, it is almost always because the wood is taking finish up at a different rate.  The can be soft spots, especially along growth rings that will absorb the finish so you get an uneven top coat.  Even though the acacia is a very hard wood, it is very sensitive to this problem,  Because the HBFP is a very thin (high alcohol) finish it is also prone to this problem.  A sealing layer is the best way to address this problem.

The HBFP will evaporate very quickly then you apply it, there needs to be a good base layer.  The Walnut Finishing Oil (DOC-106) used as a sanding lubricant, produces not only a better surface but you get a sealed surface when you friction polish the WFO before applying the film finish.  

A sanding sealer can also help.  If I have both light and dark wood in a project I don't use the WFO (to keep the colors clear).  I use a 1# cut of shellac to seal the surface, Bulls Eye Clear Coat Shellac diluted 1:1 with alcohol.  Spray on a couple of coats and repeat your last grit of sandpaper.  Slow down the lathe to about 300 rpm for the first coat of HBFP to allow coverage without evaporation to get a good smooth base layer.  

I keep a spray bottle of this by the lathe for sealing but also for tear out.  Sometime stiffening the wood just a little bit will make the cut much cleaner and reduce sanding time.

Waterproof and wood don't usually go together.  Even spar varnish isn't really waterproof.  What I can tell you is that protection of the wood, not necessarily the finish, is the goal.  Using Walnut Finishing Oil as a sanding lubricant will push the protection into the wood as you sand.  This will make for a more stable surface for any finish.  Surface is everything when it comes to finishing. Use of a sanding lubricant, sanding to finer grits of sandpaper will both add to the durability of the finish.  

There are a couple of ways to look at the duck call specifically.  A friction polished walnut oil finish is the most durable but not so much shine.  A coat of paste wax will give a better shine and is easily redone with use.  PENS PLUS has been a good finish for duck calls (see Buffalo Duck Calls ), great shine and good durability. Paste wax retouching is possible here too.  It's a piece of wood, not plastic.

The High Build Friction Polish is very high in alcohol so it dries quickly.  When applying the first coat, slow the lathe down to about 300 rpm and apply.  After the first coat has been buffed, there is a smooth, sealed surface on which to apply the remaining coats to produce that great depth of finish.  If slowing the lathe doesn't solve the problem, add a little walnut oil to the High Build Friction Polish on the towel before applying.  This will extend the open time of the finish.

The stainless steel part is a mystery to me.  Perhaps they meant steel wool.  Steel wool would clean the surface of patina, sort of an oil material that forms with time. Use 0000 steel wool.  You will see the surface loose it's shine and just take on a cleaner look as you rub.  Make sure no steel wool fibers get stuck in the surface

I can sent you two ways here: 

• Clean the surface with steel wool then use my paste wax (DOC-105) Walnut Oil Microcrystal Paste Wax to restore the surface.  It will put oil back into the wood and a good wax topcoat on the surface.  The Microcrystal wax is the best choice for this job. Buff the wood with a soft cloth, or use a mechanical buffer to restore shine.  
   
• Recommend you get some Renaissance paste wax, a product made for this job.  No need for steel wool. The solvent will strip away the patina and leave a microcrystal wax topcoat on the wood.  It won't add back oil into the wood and make sure you wear nitrile gloves or make sure you don't get it on your skin and work in a ventilated space. Buff for shine.  Don't use the bowl until you can no longer smell the solvent on the wood, about three days or less.  This sounds dire but it is safe.  You just need to know what to do.