Recarbonation? Is it possible?

It is possible to force carbonate a beer right before drinking it, but the carbonation level tends to come off overcarbonated when you do that (same concept as force carbonating with a soda stream, although there are better tools to use than the actual sodastream). It's hard to control the mouthfeel this way.

If you are asking about where or not you can open the bottles up and just add more sugar, i'd recommend not doing it. Any number of things could go wrong with that, such as oxidation or bottle bombs, leading to some weird, off-flavors. If you measure the sugar right, it'll carbonate, but it likely won't taste good (although I've heard success stories, too, so you may want to risk it.

 

If you're sure they are under-carbonated because of a bad seal (and not because they never carbonated), then yes, you could add carefully measured sugar and re-carbonate. But what assurance do you have that the seal will be better this time?

 

I don't have an opinion about whether you can try to re-carbonate, but I will offer that your problem may not be the caps on certain bottles but rather inconsistent carbonation because of not mixing the priming sugar well enough in your bottling bucket. You may want to revisit your bottling procedure before your next batch to correct this problem if you suspect that may be the reason for your lack of carbonation in some bottles. (Especially if you discover that some bottles are over-carbed.)

 

And if the PET bottles were filled either first or last, it would appear to be a bottle issue rather than a priming issue. Insufficient mixing was my first thought too.

 

No, I think if bottle types were grouped for filling, then that could point to inconsistent mixing, but it could also be an indicator that the bottles/caps are to blame. If the bottle types were mixed up when filled in random bottle styles, and if only the PET bottles have the carbonation issue, then that would confirm the bottles are the culprit. But I think the OP has to decide if the mixing of the sugar is a possibility and fix that process if the answer is yes. If none of the bottles are over-carbed, then that would also point to the bottles as a reason.

 

Are we sure that the caps on the pet bottles are not CO2 permeable? My physical chemist friend kept of having failed experiments due to CO2 going through the cap material of the bottles he was working with.

But, if these are those plastic bottles they make for home brewing, you may want to review your bottling technique and make sure you are mixing properly. If they are not carbonated decently in another week, you certainty could add sugar. Or a carb drop. If I am not mistaken, don't they make screw on attachments for bottles so they can be force carbed?

 

The one time I bottled in PET I had no issues with carbonation across two different brands of seltzer water bottles YMMV

 

Kegging is also a nice direction to take if you have the means (space, and money). It sure is nice to fill one keg instead of 50 bottles. No capping. Sanitation is so much faster.