Can I Refinish Over Existing Finish?

I have a wooden kitchen table and a wooden windowsill that I would like to refinish because they have losed most of the shine and I need to put a coat of finish over their surface to protect them. Their condition are not bad; therefore, I don't want to remove all the existing finish and refinish them. I am thinking of putting a coat of the same finish over the existing finish. Can I do that?
Based on the solvent-test using paint thinner, I believe the finish on the kitchen table is varnish. Can I simply rub a coat of varnish over the surface? Do I need to sand the surface first?
Based on the solvent-test using lacquer thinner, I believe the finish on the windowsill is lacquer. Can I simply rub a coat of lacquer over the surface? Do I need to sand the surface first? I hope that I don't need to sand the surface; otherwise, I will need to re-stain the whole thing.
Thanks in advance for any info.
Jay Chan
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snipped-for-privacy@hotmail.com wrote:

Sure. Just clean well first and - if necessary - kill any shine by sanding.
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dadiOH
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snipped-for-privacy@hotmail.com wrote:
I neglected your other questions...

Generally best but only really necessary if it is shiny or the varnish is bad. ______________

No. The new coat will slightly dissolve the existing and merge with it. ____________
In both cases, be sure the surface is free of grit & grime.
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dadiOH
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dadiOH wrote:

Thanks for the encouraging info. Now, I can proceed to refinish the kitchen table and the windowsill.
Jay Chan
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