Q1-How often can I use liquid fire in my bathroom sink?

Only a few times, when lines are clogged and water doesn’t drain or drains very slow, maybe when none of the traditional methods works. Liquid fire drain cleaners are usually high acids and they can eat or corrode any metal components if used often. In moderation they are safe for Septic Tank Systems. They should never be used in conjugation with any other chemical or drain cleaner. It is recommended to read the directions for usage carefully.

Q-2: I used Liquid Fire on tub and a bit backed up and removed the surface shine finish of the tub.  Is there something that we can apply to tub to return surface finish?

Interesting Reads on DIY refinishing the surface of the Tub. : http://www.wikihow.com/Refinish-Bathtubs and This http://www.thisoldhouse.com/toh/article/0,,197046,00.html

And you might be interested in taking a look at this kit

AquaFinish Bathtub and Tile Refinishing Kit: http://www.amazon.com/gp/product/B00642JDTM?m=A17LV7MCB6JG1B&qid=1453122277&ref_=sr_1_1&s=merchant-items&sr=1-1

Q-3: What are common signs of clogged drains?

The most common signs of a clogged drain in your home could be:

1. Experiencing slow water draining for more than 30 minutes and for continuously 2-3 days in your tub or sink, is one good indication of a clogged pipe or drain.
2. Toilet flushing results in water backing up and you see a lot of water still in the tub and staying for more than 2 mins, this is also a clear sign of a clogged sewer drain and needs immediate attention as ignoring this problem can be costly  
3. Any foul smell around the sink and any drain pipe or gurgling sound while flushing or draining is a telling sign of impending drainage blockage
4. The above signs, if monitored and checked in time can save you from costly clogged drain repairs.
   

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Q-4 Are liquid fire drain cleaner safe on PVC pipes

Most drain cleaners are water based acids or alkalis and are tested to be safe for usage on PVC pipes in designated quantities.

Knowledgebase: Links to a few datasheet

MATERIAL SAFETY DATA SHEET “LIQUID FIRE” DRAIN LINE OPENER

Find it here : 

1. http://www.ruralking.com/media/blfa_files/liquid_fire_hazard_sheet_1.pdf
2. http://www.tmssouth.com/downloads/liquid%20fire%20msds1.pdf